Reelin mRNA expression during mouse brain development

Reelin expression during embryonic brain development in lacertilian lizards

The expression of reelin mRNA and protein was studied during embryonic brain development in the lacertilian lizards L. viridis and L. galloti, by using radioactive in situ hybridization and immunohistochemistry. At all stages studied, high reelin expression was consistently found in the olfactory bulb, in the lateral cortex, and in neurons of the marginal zone and subplate of medial and dorsal cortical sectors. In the dorsal ventricular ridge (DVR), reelin expression was confined to deeply located, large cells which were more abundant in the caudal than the rostral part of the DVR. In the diencephalon, the ventral lateral geniculate complex and the perirotundal were strongly positive, whereas other nuclei were mostly negative. High reelin signal was associated with some layers in the tectum, with the torus semicircularis, cerebellar granule cell layers, and the ventral horn of the spinal cord. A more moderate signal was detected in the septal nuclei, striatum, retina, habenular nuclei, preoptic and periventricular hypothalamic components, and in reticular nuclei of the mid- and hindbrain. The medial and dorsal cortical plate and Purkinje cells were reelin-negative but expressed disabled-1 (Dab1) mRNA. When they are compared with reelin expression during mammalian brain development, our data reveal an evolutionarily conserved canvas of reelin expression, as well as significant differences, particularly in developing cortical fields. The developing lizard cortex differs from that of turtles, birds, crocodiles, and mammals in that it displays heavy reelin expression not only in neurons of the marginal zone that might be homologous to mammalian Cajal-Retzius cells, but also in subplate neurons. This difference in the pattern of reelin expression suggests that the elaborate radial organization of the lacertilian cortical plate, somewhat reminiscent of its mammalian counterpart, results from evolutionary convergence. Our data lend support to the hypothesis that the reelin signaling pathway played a significant role during cortical evolution.

The paleocortical ventricle is the origin of reelin-expressing neurons in the marginal zone of the foetal human neocortex

The subpial granular layer (SGL) is a transient cell layer in the cortical marginal zone during the period of neuronal migration into the cortical plate. The origin of the SGL has been studied by immunocytochemistry for calretinin (CR) and reelin in human foetuses from 11 to 40 gestational weeks (GW). At 11 GW, the paleocortical ventricle, a rostral dilatation of the lateral ventricle, gives rise to two fountainheads: a medial fountainhead provides neurons for the marginal zone (MZ) of the rostral cortex and rostral hippocampal rudiment, while multiple cell streams migrate from a lateral fountainhead into the MZ of the paleocortex and insula. The latero-medial gradient of neuronal packing density in the neocortical MZ indicates that migration extends farther into the neocortex. Neurons express CR already in the retrobulbar ventricular zone; they express reelin only as they approach the MZ of the paleocortex and rostral archicortex. At 16/17 GW, large numbers of CR-immunoreactive granule cells originate from the same fountainheads, and then direct medially, toward the surface of the anterior perforated substance, and laterally, into the paleocortical MZ, from where they continue into the neocortical SGL following a ventrolateral to dorsomedial gradient. From 13 to 18 GW, reelin is expressed by a subpopulation of granule cells and by Cajal-Retzius-like neurons. By 22 GW, the paleocortical ventricle undergoes regression and no longer supplies the SGL. Our results show that the paleocortical ventricle gives rise to a stream of neurons which extends over the cortical MZ as the subpial granular layer. The fact that SGL derivatives express reelin suggests that this transient cell layer may play a significant role in the establishment of the complex cytoarchitecture of the cerebral cortex.

Reelin mRNA expression during embryonic brain development in the turtle Emys orbicularis

The expression of reelin messenger ribonucleic acid (mRNA) was studied during embryonic brain development in the turtle Emys orbicularis, by using radioactive in situ hybridization. A high expression was consistently found in the olfactory bulb and in a few neurons in the marginal zone and, to a lesser extent, in the subplate of the dorsal and medial cortical sectors. In the diencephalon, the ventral division of lateral geniculate nuclei and the prospective reticular thalamic nuclei were strongly positive. High reelin signal was also associated with some layers of the tectum and with the external granule cell layer of the cerebellum. A more moderate signal was detected in the septal nuclei, striatum, dorsal ventricular ridge, retina, habenular nuclei, and hypothalamus, and in some reticular nuclei of the midbrain and hindbrain and in ventral spinal cord. The cortical plate, basal forebrain, amygdala, and tegmentum were weakly labeled. When they are compared to reelin expression during mammalian brain development, our data reveal an evolutionarily conserved canvas of reelin expression and significant differences, particularly in developing cortical fields. Most significantly, the developing turtle cortex does not display the heavy reelin expression in subpial Cajal-Retzius cells that is so typical of its mammalian counterpart. Given the key role of reelin in laminar cortical development, our data suggest that the increase in the number of reelin-producing cells and/or the amplification of reelin expression in the cortical marginal zone might have been a driving factor during the evolution of the laminated cerebral cortex from stem reptiles to mammals, as indicated in previous comparative analyses.

Thyroid hormone regulates reelin and dab1 expression during brain development

The reelin and dab1 genes are necessary for appropriate neuronal migration and lamination during brain development. Since these processes are controlled by thyroid hormone, we studied the effect of thyroid hormone deprivation and administration on the expression of reelin and dab1. As shown by Northern analysis, in situ hybridization, and immunohistochemistry studies, hypothyroid rats expressed decreased levels of reelin RNA and protein during the perinatal period [embryonic day 18 (E18) and postnatal day 0 (P0)]. The effect was evident in Cajal-Retzius cells of cortex layer I, as well as in layers V/VI, hippocampus, and granular neurons of the cerebellum. At later ages, however, Reelin was more abundant in the cortex, hippocampus, cerebellum, and olfactory bulb of hypothyroid rats (P5), and no differences were detected at P15. Conversely, Dab1 levels were higher at P0, and lower at P5 in hypothyroid animals. In line with these results, reelin RNA and protein levels were higher in cultured hippocampal slices from P0 control rats compared to those from hypothyroid animals. Significantly, thyroid-dependent regulation of reelin and dab1 was confirmed in vivo and in vitro by hormone treatment of hypothyroid rats and organotypic cultures, respectively. In both cases, thyroid hormone led to an increase in reelin expression. Our data suggest that the effects of thyroid hormone on neuronal migration may be in part mediated through the control of reelin and dab1 expression during brain ontogenesis.

Migration defects of cdk5(-/-) neurons in the developing cerebellum is cell autonomous

Cyclin-dependent kinase 5 (Cdk5) is a member of the family of cell cycle-related kinases. Previous neuropathological analysis of cdk5(-/-) mice showed significant changes in CNS development in regions from cerebral cortex to brainstem. Among the defects in these animals, a disruption of the normal pattern of cell migrations in cerebellum was particularly apparent, including a pronounced abnormality in the location of cerebellar Purkinje cells. Complete analysis of this brain region is hampered in the mutant because most of cerebellar morphogenesis occurs after birth and the cdk5(-/-) mice die in the perinatal period. To overcome this disadvantage, we have generated chimeric mice by injection of cdk5(-/-) embryonic stem cells into host blastocysts. Analysis of the cerebellum from the resulting cdk5(-/-) left arrow over right arrow cdk5(+/+) chimeric mice shows that the abnormal location of the mutant Purkinje cells is a cell-autonomous defect. In addition, significant numbers of granule cells remain located in the molecular layer, suggesting a failure to complete migration from the external to the internal granule cell layer. In contrast to the Purkinje and granule cell populations, all three of the deep cerebellar nuclear cell groupings form correctly and are composed of cells of both mutant and wild-type genotypes. Despite similarities of the cdk5(-/-) phenotype to that reported in reeler and mdab-1(-/-) (scrambler/yotari) mutant brains, reelin and disabled-1 mRNA were found to be normal in cdk5(-/-) brain. Together, the data further support the hypothesis that Cdk5 activity is required for specific components of neuronal migration that are differentially required by different neuronal cell types and by even a single neuronal cell type at different developmental stages.

Low resting potential and postnatal upregulation of NMDA receptors may cause Cajal-Retzius cell death

Using in situ patch-clamp techniques in rat telencephalic slices, we have followed resting potential (RP) properties and the functional expression of NMDA receptors in neocortical Cajal-Retzius (CR) cells from embryonic day 18 to postnatal day 13, the time around which these cells normally disappear. We find that throughout their lives CR cells have a relatively depolarized RP (approximately -50 mV), which can be made more hyperpolarized (approximately -70 mV) by stimulation of the Na/K pump with intracellular ATP. The NMDA receptors of CR cells are subjected to intense postnatal upregulation, but their similar properties (EC50, Hill number, sensitivity to antagonists, conductance, and kinetics) throughout development suggest that their subunit composition remains relatively homogeneous. The low RP of CR cells is within a range that allows for the relief of NMDA channels from Mg2+ blockade. Our findings are consistent with the hypothesis that CR cells may degenerate and die subsequent to uncontrolled overload of intracellular Ca2+ via NMDA receptor activation by ambient glutamate. In support of this hypothesis we have obtained evidence showing the protection of CR cells via in vivo blockade of NMDA receptors with dizocilpine.

Evolutionarily conserved, alternative splicing of reelin during brain development

Reelin is the protein defective in reeler mutant mice and plays a pivotal role in brain development. However, some uncertainties remain about the relationship between reelin and the reeler phenotype. It is generally believed that reelin, secreted by specific neuronal types such as Cajal-Retzius cells, acts at short distance via the extracellular matrix on target neurons, the response of which requires the Dab1 gene product. However, the pattern of reelin expression in some structures such as olfactory bulb, retina, and spinal cord suggests that the protein might be endowed with different functions. In the present study, we identify two uncommon, evolutionarily conserved splicing events in the 3' part of the transcript that result in different forms of the protein. First, a 6-nucleotide, brain-specific microexon is skipped in about 10% of reelin RNA. In addition, an alternative polyadenylation event involving 10-25% of reelin mRNA results in secretion of a truncated protein lacking the terminal, highly basic stretch. This alternative reelin is generally expressed in the same cells as the major form, but is almost undetectable in retina and spinal cord. Both alternative splicing events are present in mouse, rat, and man, suggesting that the corresponding reelin forms are functionally important.

Development of commissural connections in the hippocampus of reeler mice: evidence of an inhibitory influence of Cajal-Retzius cells

Reelin is a large, extracellular matrix protein involved in neuronal migration and axonal growth. To analyze the contribution of Reelin to the development of the commissural projection in the hippocampus, we analyzed the ontogeny of this projection in the reeler mutant mouse. Injections of the lipophilic tracer DiI revealed many commissural fibers in the hippocampus of both reeler and control mice at P1-P2. At P5, at P12, and in the adult, the topography of commissural connections was normal in the CA1 region of reeler mice, with axons innervating the stratum radiatum and stratum oriens. In contrast, in the CA3/CA2 region, commissural fibers abnormally innervated the stratum lacunosum-moleculare and, in the dentate gyrus, some fibers were observed in the outer molecular layer. Next, we monitored the distribution of Cajal-Retzius cells in the hippocampus of reeler mutant mice and noted that the stratum lacunosum-moleculare of the CA3/CA2 region was largely devoid of Cajal-Retzius (CR) cells. Taken together, the above results indicate that in the absence of CR cells in the CA3/CA2, commissural axons abnormally grow to the stratum lacunosum-moleculare. To test this hypothesis a series of coculture experiments was performed in collagen gels, in which the CA3 axonal growth was monitored when confronted to the marginal zone. These experiments showed that the marginal zone containing CR cells exerts short-range inhibitory influences for commissural axonal growth.

Cortical bitufted, horizontal, and Martinotti cells preferentially express and secrete reelin into perineuronal nets, nonsynaptically modulating gene expression

Reelin (Reln) is a protein with some structural analogies with other extracellular matrix proteins that functions in the regulation of neuronal migration during the development of cortical laminated structures. In the cortex of adult animals, Reln is expressed primarily in gamma-aminobutyric acid (GABA)ergic neurons and is secreted into perineuronal nets. However, only 50-60% of GABAergic interneurons express Reln. We have characterized this subpopulation of cortical GABAergic neurons that expresses Reln by using two strategies: (i) a double immunolabeling procedure to determine the colocalization of Reln with neuropeptides and Ca2+-binding proteins and (ii) a combination of Golgi staining and Reln immunolabeling to determine the morphology of the rat cortical cells that store Reln. Many interneurons that express Neuropeptide Y (NPY) or somatostatin (but none of those that express parvalbumin) are Reln-immunopositive. A small population of calbindin-positive interneurons and very few calretinin-positive cells express Reln immunopositivity. Golgi staining revealed that layer I horizontal cells, layer II-V bitufted neurons, and some deep cortical layer Martinotti cells express Reln. Basket and chandelier cells are often immunopositive to parvalbumin, but never to Reln. Although Reln is secreted by GABAergic neurons, its target are not the GABA receptors, but rather may be extrasynaptically located in perineuronal nets and concerned with the modulation of neuronal plasticity. Dab1, the target adapter protein that presumably mediates transcription regulation via the extrasynaptic actions of Reln, is expressed predominantly in pyramidal neurons, but it can also be detected in a small population of GABAergic neurons that are neither horizontal nor bitufted neurons.

Reelin, the extracellular matrix protein deficient in reeler mutant mice, is processed by a metalloproteinase

Reelin is the extracellular protein defective in reeler mice. It is believed that reelin acts via the extracellular matrix to influence the development of nearby neurons, but the mechanism remains thus far unknown. In the present work, we present in vivo and in vitro evidence that reelin is cleaved. This processing did not occur in Relnrl-Orl mutant mice in which reelin is not secreted and was prevented in explant cultures by brefeldin treatment, suggesting that it takes place extracellularly or in a postendoplasmic reticulum compartment. Reelin cleavage was inhibited by zinc chelators known to inhibit metalloproteinases but was unaffected by inhibitors of serine, cysteine, or aspartate proteinases. Furthermore, reelin cleavage was insensitive to inhibitors of matrixins, neprilysin, meprin, and peptidyl dipeptidase A, suggesting that the processing enzyme belongs to a different enzyme family. This enzyme and the physiological meaning of reelin processing remain to be characterized further.

Reelin regulates the development and synaptogenesis of the layer-specific entorhino-hippocampal connections

Here we examine the role of Reelin, an extracellular protein involved in neuronal migration, in the formation of hippocampal connections. Both at prenatal and postnatal stages, the general laminar and topographic distribution of entorhinal projections is preserved in the hippocampus of reeler mutant mice, in the absence of Reelin. However, developing and adult entorhinal afferents show severe alterations, including increased numbers of misrouted fibers and the formation of abnormal patches of termination from the medial and lateral entorhinal cortices. At perinatal stages, single entorhinal axons in reeler mice are grouped into thick bundles, and they have decreased axonal branching and decreased extension of axon collaterals. We also show that the number of entorhino-hippocampal synapses is lower in reeler mice than in control animals during development. Studies performed in mixed entorhino-hippocampal co-cultures combining slices from reeler and wild-type mice indicate that these abnormalities are caused by the lack of Reelin in the target hippocampus. These findings imply that Reelin fulfills a modulatory role during the formation of layer-specific and topographic connections in the hippocampus. They also suggest that Reelin promotes maturation of single fibers and synaptogenesis by entorhinal afferents.

Neurogenesis and commitment of corticospinal neurons in reeler

In the homozygous (but not the heterozygous) reeler mutant, disruption of neuron migration leads to a major perturbation of the cortical environment that in turn could modify (1) the specification of neuronal fate and (2) the proliferation dynamics of cortical precursors. To investigate these issues, tritiated thymidine injections during cortical neurogenesis were coupled with postnatal injections of a retrograde tracer in the spinal cord to accurately measure the neurogenesis of corticospinal neurons in the heterozygous and homozygous mutant. The homozygous reeler shows (1) strict conservation of area-specific timetables of corticospinal neuron generation; (2) neurons with the appropriate birthdates show an enhanced probability of projecting to the spinal cord; (3) during early stages of corticogenesis, there is a reduced rate of neuron production followed at later stages by an increased rate of neuron production; and (4) these changes in the rate of neuron production were shown to be at least partially attributable to changes in the proportions of differentiative divisions. Taken together, our results show that in the developing cortex, the neurogenesis and specification of a given neuronal phenotype are partially controlled by the postmigratory compartment. On the other hand, neither areal identity nor the chronology of production of layer-specific neuronal phenotype seems to depend on the integrity of the cellular environment.

A new view of early cortical development

Recently, several genes that regulate the development of the cerebral cortex and are potential pharmacological targets have been cloned. Reelin, an extracellular matrix glycoprotein secreted by Cajal-Retzius cells in the marginal zone, instructs the radial organization of the cortical plate. The response of cortical plate cells to reelin requires the tyrosine kinase adaptor disabled-1 (Dab1). Cyclin-dependent kinase 5 and its activator p35 are necessary for the development of the cortical plate, probably at a later stage than reelin/Dab1. The transcription factor Tbr-1 is essential for differentiation of preplate and Cajal-Retzius cells and for formation of thalamocortical connections, while D1x-1/2 are required for tangential migration. Some neurotrophin systems such as neurotrophin 4, brain-derived neurotrophic factor, and neuregulin and its receptor ErbB are also thought to assist in the regulation of cortical development. In addition, a few genes implicated in human cortical dysplasias have been characterized. LIS1 encodes a protein related to platelet-activating factor acetyl hydrolase that is defective in lissencephaly-1 of the Miller-Dieker type, while the double cortex malformation is related to mutations of a new gene dubbed doublecortn.

Accumulation of F-spondin in injured peripheral nerve promotes the outgrowth of sensory axons

F-spondin, an extracellular matrix protein, is present in peripheral nerve during embryonic development, but its amount diminishes by birth. Axotomy of adult rat sciatic nerve, however, causes a massive upregulation of both F-spondin mRNA and protein distal to the lesion. F-spondin in the distal stump of axotomized nerve promotes neurite outgrowth of sensory neurons, as revealed by protein neutralization with F-spondin-specific antibodies. Thus, F-spondin is likely to play a role in promoting axonal regeneration after nerve injury.

Ambiguus nucleus neurons innervating the abdominal esophagus are malpositioned in the reeler mouse

To examine whether the migration of ambiguus nucleus (NA) neurons is affected in the reeler mouse, recombinant replication-deficient adenoviral vector carrying E. coli-galactosidase gene (lacZ) was injected into the abdominal esophagus of the reeler mouse and normal control at two months of age prior to 5 days of sacrifice of the animals. In the normal control, lacZ-positive neurons were found in the compact formation of the NA, whereas, in the reeler, they were scattered from the base of the fourth ventricle to the ventro-lateral margin of the medulla. The present study confirmed that NA neurons are malpositioned in the reeler mouse, suggesting that the migration of NA neurons is guided by the reelin-related protein (Reelin).

Simultaneous detection of glutamic acid decarboxylase and reelin mRNA in adult rat neurons using in situ hybridization and immunofluorescence

The combination of in situ hybridization and immunocytochemical technique is an important tool to detail the biochemical phenotype of individual neurons. In this work, we have developed a double fluorescence method to show the presence of reelin mRNA in GABAergic cells. This was achieved by demonstrating the colocalization of glutamic acid decarboxylase67, the synthesizing enzyme for GABA, with the mRNA for reelin, a novel factor involved in brain development and possibly the maintenance of the synaptic organization of layered structures in adult brain. The results demonstrated that reelin is expressed primarily in GABAergic cells in the adult rat cerebrum, but not in the cerebellum.

Regional and cellular patterns of reelin mRNA expression in the forebrain of the developing and adult mouse

The reelin gene encodes an extracellular protein that is crucial for neuronal migration in laminated brain regions. To gain insights into the functions of Reelin, we performed high-resolution in situ hybridization analyses to determine the pattern of reelin expression in the developing forebrain of the mouse. We also performed double-labeling studies with several markers, including calcium-binding proteins, GAD65/67, and neuropeptides, to characterize the neuronal subsets that express reelin transcripts. reelin expression was detected at embryonic day 10 and later in the forebrain, with a distribution that is consistent with the prosomeric model of forebrain regionalization. In the diencephalon, expression was restricted to transverse and longitudinal domains that delineated boundaries between neuromeres. During embryogenesis, reelin was detected in the cerebral cortex in Cajal-Retzius cells but not in the GABAergic neurons of layer I. At prenatal stages, reelin was also expressed in the olfactory bulb, and striatum and in restricted nuclei in the ventral telencephalon, hypothalamus, thalamus, and pretectum. At postnatal stages, reelin transcripts gradually disappeared from Cajal-Retzius cells, at the same time as they appeared in subsets of GABAergic neurons distributed throughout neocortical and hippocampal layers. In other telencephalic and diencephalic regions, reelin expression decreased steadily during the postnatal period. In the adult, there was prominent expression in the olfactory bulb and cerebral cortex, where it was restricted to subsets of GABAergic interneurons that co-expressed calbindin, calretinin, neuropeptide Y, and somatostatin. This complex pattern of cellular and regional expression is consistent with Reelin having multiple roles in brain development and adult brain function.

Cajal-Retzius cells, Reelin, and the formation of layers

Early-generated Cajal-Retzius cells in the marginal zone of the cortex synthesize and secrete the glycoprotein Reelin. The reelin gene is deleted in reeler mice, which show characteristic alterations in cortical lamination. Recent studies have shed some light on the role of Cajal-Retzius cells and Reelin in the formation of cell and fiber layers in the neocortex and hippocampus.

BDNF regulates reelin expression and Cajal-Retzius cell development in the cerebral cortex

Cajal-Retzius (CR) cells of the cerebral cortex express receptors for the neurotrophin brain-derived neurotrophic factor (BDNF) and downregulate expression of the extracellular matrix protein Reelin during early postnatal development, coincident with the onset of cortical BDNF expression. During this period, mice lacking BDNF have elevated levels of Reelin in CR cells. Acute BDNF stimulation of cortical neuron cultures and overexpression of BDNF in the developing brain of transgenic mice prior to the onset of endogenous production causes a profound, dose-dependent reduction of Reelin expression in CR cells. In addition, overexpression of BDNF produces gaps and heterotopias in the marginal zone and disorganization and aggregation of cortical CR cells and induces several other malformations, including aberrant cortical lamination, similar to the phenotype of reeler mutant mice, which lack Reelin. These results demonstrate a role for BDNF on cortical CR cells and identify Reelin as a direct effector of this neurotrophin during brain development.

A panel of monoclonal antibodies against reelin, the extracellular matrix protein defective in reeler mutant mice

Reelin, the extracellular matrix protein defective in reeler mutant mice, plays a key role during brain development. We therefore raised antibodies directed against various reelin epitopes in order to facilitate biochemical and cell biological studies of this important molecule. Homozygous reeler mice with a large deletion of most of the reelin gene were immunized with fusion proteins and carrier-coupled peptides corresponding to parts of the reelin sequence. Monoclonal antibodies were produced using classical procedures, screened using ELISA and-or western blot prepared with the antigen, and tested by immunohistochemistry and immunoprecipitation assays to detect endogenous reelin. The labeling of Cajal-Retzius cells in the embryonic mouse telencephalon was selected as criterion for positivity in immunohistochemistry. A total of 11 monoclonal antibodies were obtained, providing useful additions to the widely used antibody CR-50. Five are directed against the N-terminal part of reelin, among which three recognize the region that has significant similarity with F-spondin, and two are specific for hinge region located downstream from the F-spondin similarity region and upstream from the reelin repeats. Six antibodies are directed against the C-terminal part of reelin, among which one anti-peptide antibody recognizes the highly basic C-terminal segment. Antibodies against the N-terminal region stain well in immunohistochemistry. By comparison, the labeling of embryonic Cajal-Retzius cells with antibodies directed against the C-terminal region is weaker, suggesting that this part of the molecule might be modified or not be as readily accessible in the tissue as the N-terminus.

Reelin is preferentially expressed in neurons synthesizing gamma-aminobutyric acid in cortex and hippocampus of adult rats

During embryonic development of brain laminated structures, the protein Reelin, secreted into the extracellular matrix of the cortex and hippocampus by Cajal-Retzius (CR) cells located in the marginal zone, contributes to the regulation of migration and positioning of cortical and hippocampal neurons that do not synthesize Reelin. Soon after birth, the CR cells decrease, and they virtually disappear during the following 3 weeks. Despite their disappearance, we can quantify Reelin mRNA (approximately 200 amol/ g of total RNA) and visualize it by in situ hybridization, and we detect the translated product of this mRNA by immunocytochemistry preferentially in gamma-aminobutyric acid (GABA)ergic neurons of adult rat cortex and hippocampus. In adult rat cerebellum, Reelin is expressed in glutamatergic neurons (granule cells). The translated product of this mRNA is readily exported from the granule cell somata to the parallel fibers, where it has been detected by electron microscopy in axon terminals located presynaptically to Purkinje cell dendrites.

Reeler: new tales on an old mutant mouse

Neurological mouse mutants provide an opportunity to dissect the complex mechanisms that underlie vertebrate brain development. Advances in genetic technologies have permitted the identification of genes disrupted in many mutants, allowing a molecular interpretation of the phenotypes. For several decades, the spontaneous mutant mouse reeler has been used as a model for the analysis of the development of laminated brain structures. In this ataxic mutant, the migration of many neurons is aberrant, resulting in disrupted cellular organization. Recently, reelin, the gene disrupted in the reeler mouse, has been identified, reelin encodes a novel extracellular molecule that controls neural cell positioning through mechanisms that are not yet completely understood. Analysis of the expression pattern and the properties of the reelin gene product (Reelin) suggests models for its function during brain development. Furthermore, the recent identification of genes that may function in the Reelin signaling pathway advances our knowledge of the molecular basis of neuronal migration.

New directions for neuronal migration

Analysis of genetic mutations that lead to abnormal migration and layer formation in the developing cerebral cortex of mice and humans has led to important new discoveries regarding the molecular mechanisms that underlie these processes. Genetic manipulation and experimental analysis have demonstrated significant tangential migrations of cortical neurons, some arriving from very distant noncortical sites.

Genomic organization of the mouse reelin gene

Reelin is the protein defective in reeler mice, an extensively studied model of brain development. The reelin gene (symbol Reln) codes for a protein of the extracellular matrix that contains eight successive repeats of 350 to 390 amino acids. In this work, we describe the genomic structure of the mouse reelin gene and the 5'-flanking genomic DNA sequences. The reelin gene is composed of 65 exons spread over approximately 450 kb of genomic DNA. We identified different reelin transcripts, formed by alternative splicing of a microexon as well as by use of two different polyadenylation sites. All splice sites conform to the GT-AG rule, except for the splice donor site of intron 30, which is GC instead of GT. A processed pseudogene is present in intron 42. Its nucleotide sequence is 86% identical to the sequence of the rat RDJ1 cDNA, which codes for a DnaJ-like protein of the Hsp40 family. Comparison of 8 intron positions in mouse and human reelin genes reveals a highly conserved genomic structure, suggesting a similar structure of the whole gene in both species. We identified two transcription start sites embedded within a CpG. The promoter region contains putative recognition sites for the transcription factors Sp1 and AP2 but lacks TATA and CAAT boxes. The presence of tandemly repeated regions in the Reelin protein suggests that gene duplication events occurred during evolution. By comparison of the amino acid sequences of the eight repeats and the positions of introns, we suggest a model for the evolution of the repeat coding portion of the reelin gene from a putative ancestral minigene.

A truncated Reelin protein is produced but not secreted in the 'Orleans' reeler mutation (Reln[rl-Orl])

Reelin is the protein defective in reeler mutant mice [I. Bar, C. Lambert de Rouvroit, I. Royaux, D.B. Krizman, C. Dernoncourt, D. Ruelle, M.C. Beckers, A.M. Goffinet, A YAC contig containing the reeler locus with preliminary characterization of candidate gene fragments, Genomics 26 (1995) 543-549; G. D'Arcangelo, G.G. Miao, S.C. Chen, H.D. Soares, J.I. Morgan, T. Curran, A protein related to extracellular matrix proteins deleted in the mouse mutant reeler, Nature 374 (1995) 719-723; S. Hirotsune, T. Takahara, N. Sasaki, K. Hirose, A. Yoshiki, T. Ohashi, M. Kusakabe, Y. Murakami, M. Muramatsu, S. Watanabe, K. Nakao, M. Katsuki, Y. Hayashizaki, The reeler gene encodes a protein with an EGF-like motif expressed by pioneer neurons, Nature Genet. 10 (1995) 77-83]. In the Orleans allele of reeler (symbol: Reln[rl-Orl]), a 220 nucleotide deletion is present in the 3' region of the Reelin message, resulting in a frame shift with production of a predicted protein amputated from its C-terminal amino acids. In this study, we first show that the predicted truncated protein indeed exists in Orleans reeler mice, using several anti-Reelin antibodies. Three antibodies are directed against epitopes located in the N-terminal region of the protein, namely: monoclonal antibody CR-50 [M. Ogawa, T. Miyata, K. Nakajima, K. Yagyu, M. Seike, K. Ikenaka, H. Yamamoto, K. Mikoshiba, The reeler gene-associated antigen on Cajal-Retzius neurons is a crucial molecule for laminar organization of cortical neurons, Neuron 14 (1995) 899-912] (epitope region between Reelin residues 251-407), monoclonal antibody G10 (epitope located between amino acids 199 and 244) and the polyclonal antipeptide rp4 (positions 381-399). A fourth antibody, antipeptide rp5, reacts with the C-terminal (3443-3461) Reelin sequence. In normal embryos, all four antibodies stained cells in the marginal zone with features of Cajal-Retzius cells. While N-terminal specific antibodies detected Reelin immunoreactivity in mouse embryos homozygous for the reeler-Orleans mutation, no staining was obtained with the rp5 antibody, showing the presence of a truncated protein. Moreover, although Reelin could be detected at the surface of living Cajal-Retzius cells of normal mice, it was not revealed after vital staining of embryonic cortex from Orleans reeler mice. These results indicate that the C-terminal region of Reelin is essential for its secretion and suggest that the Orleans reeler phenotype is due to defective Reelin secretion rather than to secretion of an inactive protein.

Regulation of Purkinje cell alignment by reelin as revealed with CR-50 antibody

Cerebellar Purkinje cells are generated in the ventricular zone, migrate outward, and finally form a monolayer in the cortex. In reeler mice, however, most Purkinje cells cluster abnormally in subcortical areas. Reelin, the candidate reeler gene product recognized by the CR-50 monoclonal antibody, is concentrated in a cortical zone along which Purkinje cells are aligned linearly, implying that it may regulate their alignment. We used an in vitro system and a transplantation approach to analyze the function of Reelin. Explant culture for 7 d of cerebella isolated from wild-type and reeler mice at embryonic day 13 (E13) reproduced in a phenotype-dependent manner the two distinct arrangement patterns (linear vs clustered) of Purkinje cells. Extensive CR-50 binding to wild-type explants converted the linear pattern into a reeler-like, clustered pattern. On the other hand, when reeler explants lacking Reelin were crowned with an artificial layer of Reelin+ granule cells, some Reelin molecules were distributed into a superficial zone of the reeler explants, and Purkinje cells formed a linear pattern along the Reelin-rich overlay. This "rescue" effect was also inhibited by CR-50. Hence, Reelin is involved in the Purkinje cell alignment, and the lack of this activity may explain the malformation in reeler cerebella. We further injected Reelin+ granule cells into the fourth ventricle of E12-13 mice. Extensive incorporation of the injected Reelin+ cells into the ventricular zone, but not of Reelin- cells, forced Purkinje cells of the host cerebella to form an aberrant layer, suggesting that premigratory Purkinje cells may already be responsive to Reelin or Reelin-related signals.

Neuronal heterotopias in the developing cerebral cortex produced by neurotrophin-4

The marginal zone (MZ) of embryonic neocortex is crucial to its normal development. We report that neurotrophin-4 (but not NT3 or NGF), applied to embryonic rodent cortex in vitro or in vivo, produces heterotopic accumulations of neurons in the MZ. Although heterotopia production is TrkB mediated, BDNF is >10-fold less effective than NT4. Heterotopic neurons have the same birth date and phenotype as normal MZ neurons; they are not the result of NT4-induced proliferation or rescue from apoptosis. We suggest that NT4 causes excess neurons to migrate into the MZ and thus may play a role in normal MZ formation as well as in the pathogenesis of certain human cortical dysplasias.