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Crayfish, as an invertebrate, relies only on the innate immune system to resist external pathogens. In this study, a molecule containing a single Reeler domain was identified from red swamp crayfish Procambarus clarkii (named as PcReeler). Tissue distribution analysis showed that PcReeler was highly expressed in gills and its expression was induced by bacterial stimulation. Inhibiting the expression of PcReeler by RNA interference led to a significant increase in the bacterial abundance in the gills of crayfish, and a significant increase in the crayfish mortality. Silencing of PcReeler influenced the stability of the microbiota in the gills revealed by 16S rDNA high-throughput sequencing. Recombinant PcReeler showed the ability to bind microbial polysaccharide and bacteria and to inhibit the formation of bacterial biofilms. These results provided direct evidence for the involvement of PcReeler in the antibacterial immune mechanism of P. clarkii.

Reelin (Reln) and Disabled-1 (Dab1) participate in the Reln-signaling pathway and when either is deleted, mutant mice have the same spinally mediated behavioral abnormalities, increased sensitivity to noxious heat and a profound loss in mechanical sensitivity. Both Reln and Dab1 are highly expressed in dorsal horn areas that receive and convey nociceptive information, Laminae I-II, lateral Lamina V, and the lateral spinal nucleus (LSN). Lamina I contains both projection neurons and interneurons that express Neurokinin-1 receptors (NK1Rs) and they transmit information about noxious heat both within the dorsal horn and to the brain. Here, we ask whether the increased heat nociception in Reln and dab1 mutants is due to incorrectly positioned dorsal horn neurons that express NK1Rs. We found more NK1R-expressing neurons in Reln-/- and dab1-/- Laminae I-II than in their respective wild-type mice, and some NK1R neurons co-expressed Dab1 and the transcription factor Lmx1b, confirming their excitatory phenotype. Importantly, heat stimulation in dab1-/- mice induced Fos in incorrectly positioned NK1R neurons in Laminae I-II. Next, we asked whether these ectopically placed and noxious-heat responsive NK1R neurons participated in pain behavior. Ablation of the superficial NK1Rs with an intrathecal injection of a substance P analog conjugated to the toxin saporin (SSP-SAP) eliminated the thermal hypersensitivity of dab1-/- mice, without altering their mechanical insensitivity. These results suggest that ectopically positioned NK1R-expressing neurons underlie the heat hyperalgesia of Reelin-signaling pathway mutants, but do not contribute to their profound mechanical insensitivity.

The Reelin-signaling pathway regulates neuronal positioning during embryonic development. Reelin, the extracellular matrix protein missing in reeler mutants, is secreted by neurons in laminae I, II and V, binds to Vldl and Apoer2 receptors on nearby neurons, and tyrosine phosphorylates the adaptor protein Disabled-1 (Dab1), which activates downstream signaling. We previously reported that reeler and dab1 mutants had significantly reduced mechanical and increased heat nociception. Here we extend our analysis to chemical, visceral, and cold pain and importantly, used Fos expression to relate positioning errors in mutant mouse dorsal horn to changes in neuronal activity. We found that noxious mechanical stimulation-induced Fos expression is reduced in reeler and dab1 laminae I-II, compared to wild-type mice. Additionally, mutants had fewer Fos-immunoreactive neurons in the lateral-reticulated area of the deep dorsal horn than wild-type mice, a finding that correlates with a 50% reduction and subsequent mispositioning of the large Dab1-positive cells in the mutant lateral-reticulated area. Furthermore, several of these Dab1 cells expressed Fos in wild-type mice but rarely in reeler mutants. By contrast, paralleling the behavioral observations, noxious heat stimulation evoked significantly greater Fos expression in laminae I-II of reeler and dab1 mutants. We then used the formalin test to show that chemical nociception is reduced in reeler and dab1 mutants and that there is a corresponding decrease in formalin-induced Fos expression. Finally, neither visceral pain nor cold-pain sensitivity differed between wild-type and mutant mice. As differences in the nociceptor distribution within reeler and dab1 mutant dorsal horn were not detected, these differential effects observed on distinct pain modalities suggest that dorsal horn circuits are organized along modality-specific lines.

Mice missing either Reelin or Disabled‐1 (Dab1) exhibit dorsal horn neuronal positioning errors and display heat hypersensitivity and mechanical insensitivity. Reelin binds its receptors, apolipoprotein E receptor 2 and very low‐density lipoprotein receptor, leading to the recruitment and phosphorylation of Dab1 and activation of downstream pathways that regulate neuronal migration. Previously, we reported that 70% of Dab1 laminae I–II neurons co‐expressed LIM‐homeobox transcription factor 1‐beta (Lmx1b). Here, we asked whether Reelin‐expressing dorsal horn neurons co‐express Lmx1b, are mispositioned in dab1 mutants, and contribute to nociceptive abnormalities. About 90% of Reelin‐labeled neurons are Lmx1b‐positive in laminae I–II, confirming that most Reelin and Dab1 neurons are glutamatergic. We determined that Reelin‐Lmx1b and Dab1‐Lmx1b dorsal horn neurons are separate populations, and together, comprise 37% of Lmx1b‐positive cells within and above the Isolectin B4 (IB4) layer in wild‐type mice. Compared to wild‐type mice, dab1 mutants have a reduced area of laminae I–II outer (above the IB4 layer), more Reelin‐Lmx1b neurons within the IB4 layer, and fewer Reelin‐Lmx1b neurons within the lateral reticulated area of lamina V and lateral spinal nucleus. Interestingly, both Reelin‐ and Dab1‐labeled dorsal horn neurons sustain similar positioning errors in mutant mice. After noxious thermal and mechanical stimulation, Reelin, Lmx1b, and Reelin‐Lmx1b neurons expressed Fos in laminae I–II and the lateral reticulated area in wild‐type mice and, therefore, participate in nociceptive circuits. Together, our data suggest that disruption of the Reelin‐signaling pathway results in neuroanatomical abnormalities that contribute to the nociceptive changes that characterize these mutant mice. [ABSTRACT FROM AUTHOR]

The motor deficit of the reeler mutants has largely been considered cerebellum related, and the developmental consequences of the cortex on reeler motor behavior have not been examined. We herein showed that there is a behavioral consequence to reeler mutation in models examined at cortex-dependent bimanual tasks that require forepaw dexterity. Using intracortical microstimulation, we found the forelimb representation in the motor cortex was significantly reduced in the reeler. The reeler cortex required a significantly higher current to evoke skeletal muscle movements, suggesting the cortical trans-synaptic propagation is disrupted. When the higher current was applied, the reeler motor representation was found preserved. To elucidate the influence of cerebellum atrophy and ataxia on the obtained results, the behavioral and neurophysiological findings in reeler mice were reproduced using the Disabled-1 (Dab1) cKO mice, in which the Reelin-Dab1 signal deficiency is confined to the cerebral cortex. The Dab1 cKO mice were further assessed at the single-pellet reach and retrieval task, displaying a lower number of successfully retrieved pellets. It suggests the abnormality confined to the cortex still reduced the dexterous motor performance. Although possible muscular dysfunction was reported in REELIN-deficient humans, the function of the reeler forelimb muscle examined by electromyography, morphology of neuromuscular junction and the expression level of choline acetyltransferase were normal. Our results suggest that the mammalian laminar structure is necessary for the forepaw skill performance and for trans-synaptic efficacy in the cortical output. [ABSTRACT FROM AUTHOR]

Proper neuronal migration and laminar formation during corticogenesis is essential for normal brain function. Disruption of these developmental processes is thought to be involved in the pathogenesis of some neuropsychiatric conditions. Especially, Reelin, a glycoprotein mainly secreted by the Cajal-Retzius cells and a subpopulation of GABAergic interneurons, has been shown to play a critical role, both during embryonic and postnatal periods. Indeed, animal studies have clearly revealed that Reelin is an essential molecule for proper migration of cortical neurons and finally regulates the cell positioning in the cortex during embryonic and early postnatal stages; by contrast, Reelin signaling is closely involved in synaptic function in adulthood. In humans, genetic studies have shown that the reelin gene (RELN) is associated with a number of psychiatric diseases, including schizophrenia, bipolar disorder and autistic spectrum disorder. Indeed, Reln haploinsufficiency has been shown to cause cognitive impairment in rodents, suggesting the expression level of the Reelin protein is closely related to the higher brain functions. However, the molecular abnormalities in the Reelin pathway involved in the pathogenesis of psychiatric disorders are not yet fully understood. In this paper, we review the current progress in the understanding of the Reelin functions that could be related to the pathogenesis of psychiatric disorders. Furthermore, we discuss the basis for selecting Reelin and molecules in its downstream signaling pathway as potential therapeutic targets for psychiatric illnesses.

The reeler mouse mutant has long served as a primary model to study the development of cortical layers, which is governed by the extracellular glycoprotein reelin secreted by Cajal-Retzius cells. Because layers organize local and long-range circuits for sensory processing, we investigated whether intracortical connectivity is compromised by reelin deficiency in this model. We generated a transgenic reeler mutant (we used both sexes), in which layer 4-fated spiny stellate neurons are labeled with tdTomato and applied slice electrophysiology and immunohistochemistry with synaptotagmin-2 to study the circuitry between the major thalamorecipient cell types, namely excitatory spiny stellate and inhibitory fast-spiking (putative basket) cells. In the reeler mouse, spiny stellate cells are clustered into barrel equivalents. In these clusters, we found that intrinsic physiology, connectivity, and morphology of spiny stellate and fast-spiking, putative basket cells does not significantly differ between reeler and controls. Properties of unitary connections, including connection probability, were very comparable in excitatory cell pairs and spiny stellate/fast-spiking cell pairs, suggesting an intact excitation-inhibition balance at the first stage of cortical sensory information processing. Together with previous findings, this suggests that thalamorecipient circuitry in the barrel cortex develops and functions independently of proper cortical lamination and postnatal reelin signaling., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Crayfish, as an invertebrate, relies only on the innate immune system to resist external pathogens. In this study, a molecule containing a single Reeler domain was identified from red swamp crayfish Procambarus clarkii (named as Pc Reeler). Tissue distribution analysis showed that Pc Reeler was highly expressed in gills and its expression was induced by bacterial stimulation. Inhibiting the expression of Pc Reeler by RNA interference led to a significant increase in the bacterial abundance in the gills of crayfish, and a significant increase in the crayfish mortality. Silencing of Pc Reeler influenced the stability of the microbiota in the gills revealed by 16S rDNA high-throughput sequencing. Recombinant Pc Reeler showed the ability to bind microbial polysaccharide and bacteria and to inhibit the formation of bacterial biofilms. These results provided direct evidence for the involvement of Pc Reeler in the antibacterial immune mechanism of P. clarkii ., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 The Authors. Published by Elsevier Ltd.)

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., Competing Interests: The authors declare no competing financial interests., (Copyright © 2023 Honda et al.)

The Reeler heterozygous mice ( reln +/− ) are haplodeficient in the gene ( reln ) encoding for the reelin glycoprotein (RELN) and display reductions in brain/peripheral RELN similar to autistic or schizophrenic patients. Cytoarchitectonic alterations of the reln +/− brain may be subtle, and are difficult to demonstrate by current histological approaches. We analyzed the number and topological organization of the Purkinje neurons (PNs) in five vermal lobules – central (II–III), culmen (IV–V), tuber (VIIb), uvula (IX), and nodulus (X) – that process different types of afferent functional inputs in reln +/+ and reln +/− adult mice (P60) of both sexes ( n = 24). Animals were crossed with L7GFP mice so that the GFP-tagged PNs could be directly identified in cryosections. Digital images from these sections were processed with different open source software for quantitative topological and statistical analyses. Diversity indices calculated were: maximum caliper, density, area of soma, dispersion along the XZ axis, and dispersion along the YZ axis. We demonstrate: i. reduction in density of PNs in reln +/− males (14.37%) and reln +/− females (17.73%) compared to reln +/+ males; ii. that reln +/− males have larger PNs than other genotypes, and females (irrespective of the reln genetic background) have smaller PNs than reln +/+ males; iii. PNs are more chaotically arranged along the YZ axis in reln +/− males than in reln +/+ males and, except in central lobulus, reln +/− females. Therefore, image processing and statistics reveal previously unforeseen gender and genotype-related structural differences in cerebellum that may be clues for the definition of novel biomarkers in human psychiatric disorders. [ABSTRACT FROM AUTHOR]

A mutation of the reln gene gives rise to the Reeler mouse (reln-/-) displaying an ataxic phenotype and cerebellar hypoplasia. We have characterized the neurochemistry of postnatal (P0-P60) reln-/- mouse cerebella with specific attention to the intervention of cell proliferation and apoptosis in the P0-P25 interval. Homozygous reln-/- mice and age-matched controls were analyzed by immunofluorescence using primary antibodies against NeuN, calbindin, GFAP, vimentin, SMI32, and GAD67. Proliferation and apoptosis were detected after a single intraperitoneal BrdU injection and by the TUNEL assay with anti-digoxigenin rhodamine-conjugated antibodies. Quantitative analysis with descriptive and predictive statistics was used to calculate cell densities (number/mm²) after fluorescent nuclear stain (TCD, total cell density), labeling with BrdU (PrCD, proliferating cell density), or TUNEL (ApoCD, apoptotic cell density). By this approach we first have shown that the temporal pattern of expression of neuronal/glial markers in postnatal cerebellum is not affected by the Reeler mutation. Then, we have demonstrated that the hypoplasia in the Reeler mouse cerebellum is consequent to reduction of cortical size and cellularity (TCD), and that TCD is, in turn, linked to quantitative differences in the extent of cell proliferation and apoptosis, as well as derangements in their temporal trends during postnatal maturation. Finally, we have calculated that PrCD is the most important predictive factor to determine TCD in the cerebellar cortex of the mutants. These results support the notion that, beside the well-known consequences onto the migration of the cerebellar neurons, the lack of Reelin results in a measurable deficit in neural proliferation. [ABSTRACT FROM AUTHOR]

Reelin is an extracellular matrix protein that is defective in reeler mutant mice and plays a key role in the organization of architectonic patterns, particularly in the cerebral cortex. In mammals, a "reelin signal" is activated when reelin, secreted by Cajal-Retzius neurons, binds to receptors of the lipoprotein receptor family on the surface of cortical plate cells, and triggers Dab1 phosphorylation. As reelin is a key component of cortical development in mammals, comparative embryological studies of reelin expression were carried out during cortical development in non-mammalian amniotes (turtles, squamates, birds and crocodiles) in order to assess the putative role of reelin during cortical evolution. The data show that reelin is present in the cortical marginal zone in all amniotes, and suggest that reelin has been implicated in the evolution of the radial organization of the cortical plate in the synapsid lineage leading from stem amniotes to mammals, as well as in the lineage leading to squamates, thus providing an example of homoplastic evolution (evolutionary convergence). The mechanisms by which reelin instructs radial cortical organization in these two lineages seem different: in the synapsid lineage, a drastic amplification of reelin production occurred in Cajal-Retzius cells, whereas in squamates, in addition to reelin-secreting cells in the marginal zone, a second layer of reelin-producing cells developed in the subcortex. Altogether, our results suggest that the reelin-signaling pathway has played a significant role in shaping the evolution of cortical development.

A hallmark of neocortical circuits is the segregation of processing streams into six distinct layers. The importance of this layered organization for cortical processing and plasticity is little understood. We investigated the structure, function and plasticity of primary visual cortex (V1) of adult mice deficient for the glycoprotein reelin and their wild-type littermates. In V1 of rl−/− mice, cells with different laminar fates are present at all cortical depths. Surprisingly, the (vertically) disorganized cortex maintains a precise retinotopic (horizontal) organization. Rl−/− mice have normal basic visual capabilities, but are compromised in more challenging perceptual tasks, such as orientation discrimination. Additionally, rl−/− animals learn and memorize a visual task as well as their wild-type littermates. Interestingly, reelin deficiency enhances visual cortical plasticity: juvenile-like ocular dominance plasticity is preserved into late adulthood. The present data offer an important insight into the capabilities of a disorganized cortical system to maintain basic functional properties. [ABSTRACT FROM AUTHOR]

Summary: Reelin, an extracellular protein promoting neuronal migration in brain areas with a laminar architecture, is missing in the Reeler mouse (reelin −/−). Several studies indicate that the protein is also necessary for correct dendritic outgrowth and synapse formation in the adult forebrain. By transmission electron microscopy, we characterize the development and synaptic organization of the cerebellar cortex in Reeler mice and wild type control littermates at birth, postnatal day (P) 5, 7, 10 and 15. Ultrastructural analysis shows deep alterations in cortical architecture and mispositioning of the Purkinje neurons (Pns), which remain deeply embedded in a central cellular mass within the white matter, with highly immature features. Quantitative examination shows that Reeler mice display: (i) a lower density of granule cells and a higher density of Pns, from P10; (ii) a lower density of synaptic contacts between Pn dendrites and parallel or climbing fibers, from P5; (iii) a lower density of synaptic contacts between basket cells and Pns, from P5; and (iv) a lower density of mossy fiber rosettes, from P10. Our results demonstrate that Reelin profoundly affects the structure and synaptic connectivity of post-natal mouse cerebellum. [ABSTRACT FROM AUTHOR]

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 α PIX/Arhgef6 promoted translocation of Golgi cisternae into developing dendrites of hippocampal neurons. Reelin treatment further increased the α PIX-dependent effect. In turn, overexpression of exchange activity-deficient α PIX 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 α PIX 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. [ABSTRACT FROM AUTHOR]

Abstract: The Reelin-signaling pathway regulates neuronal positioning during embryonic development. Reelin, the extracellular matrix protein missing in reeler mutants, is secreted by neurons in laminae I, II and V, binds to Vldl and Apoer2 receptors on nearby neurons, and tyrosine phosphorylates the adaptor protein Disabled-1 (Dab1), which activates downstream signaling. We previously reported that reeler and dab1 mutants had significantly reduced mechanical and increased heat nociception. Here we extend our analysis to chemical, visceral, and cold pain and importantly, used Fos expression to relate positioning errors in mutant mouse dorsal horn to changes in neuronal activity. We found that noxious mechanical stimulation-induced Fos expression is reduced in reeler and dab1 laminae I-II, compared to wild-type mice. Additionally, mutants had fewer Fos-immunoreactive neurons in the lateral-reticulated area of the deep dorsal horn than wild-type mice, a finding that correlates with a 50% reduction and subsequent mispositioning of the large Dab1-positive cells in the mutant lateral-reticulated area. Furthermore, several of these Dab1 cells expressed Fos in wild-type mice but rarely in reeler mutants. By contrast, paralleling the behavioral observations, noxious heat stimulation evoked significantly greater Fos expression in laminae I–II of reeler and dab1 mutants. We then used the formalin test to show that chemical nociception is reduced in reeler and dab1 mutants and that there is a corresponding decrease in formalin-induced Fos expression. Finally, neither visceral pain nor cold-pain sensitivity differed between wild-type and mutant mice. As differences in the nociceptor distribution within reeler and dab1 mutant dorsal horn were not detected, these differential effects observed on distinct pain modalities suggest that dorsal horn circuits are organized along modality-specific lines. [Copyright &y& Elsevier]

Brain-derived neurotrophic factor (BDNF) signalling through its receptor, TrkB is known to regulate GABAergic function and glutamic acid decarboxylase (GAD) 67 expression in neurons. Alterations in BDNF signalling have been implicated in the pathophysiology of schizophrenia and as a result, they are a potential therapeutic target. Interestingly, heterozygous reeler mice (HRM) have decreased GAD67 expression in the frontal cortex and hippocampus and they exhibit many behavioural and neurochemical abnormalities similar to schizophrenia. In this study, we evaluated the potential of cysteamine, a neuroprotective compound to improve the deficits in GAD67 expression and cognitive function in HRM. We found that cysteamine administration (150 mg/kg.d, through drinking water) for 30 d significantly ameliorated the decreases in GAD67, mature BDNF and full-length TrkB protein levels found in frontal cortex and hippocampus of HRM. A significant attenuation of the increased levels of truncated BDNF in frontal cortex and hippocampus, as well as truncated TrkB in frontal cortex of HRM was also observed following cysteamine treatment. In behavioural studies, HRM were impaired in a Y-maze spatial recognition memory task, but not in a spontaneous alternation task or a sensorimotor, prepulse inhibition (PPI) procedure. Cysteamine improved Y-maze spatial recognition in HRM to the level of wide-type controls and it improved PPI in both wild-type and HRM. Finally, mice deficient in TrkB, showed a reduced response to cysteamine in GAD67 expression suggesting that TrkB signalling plays an important role in GAD67 regulation by cysteamine. [ABSTRACT FROM AUTHOR]

Neuronal migration is essential for the development of the cerebral cortex. Mutations leading to defective migration are associated with numerous brain pathologies. An important challenge in the field is to understand the intrinsic and extrinsic mechanisms that regulate neuronal migration during normal development and in disease. Many small GTPases are expressed in the central nervous system during embryonic development. Recent findings have shown that Rap1 and its downstream partners Ral, Rac and Cdc42 are involved in themaintenance of N-Cadherin at the plasma membrane which is necessary for the correct polarization of migrating neurons. The activation of Rap1 is triggered by Reelin, an extracellular protein known for its role in the organization of the cortex into layers of neurons. In the absence of Reelin, neurons exhibit a broader and irregular pattern of positioning. The prevailing model suggests that Reelin signals to neurons during the last step of their migration, a notion that is inconsistent with new data describing an effect of Reelin on early steps of migration. In regard to these recent findings I suggest a revised model, which I call the "polarity model," that further refines our understanding of the developmental function played by Reelin and its downstream small GTPases. [ABSTRACT FROM AUTHOR]

Abstract: The extracellular protein Reelin regulates radial neuronal migration in the embryonic brain, promotes dendrite outgrowth in the developing postnatal forebrain, and strengthens synaptic transmission in the adult brain. Heterozygous reeler mice expressing reduced levels of Reelin are grossly normal but exhibit behavioral and physiological abnormalities. We previously demonstrated that dendritic spine density is reduced in the developing hippocampus of these mice. In this study, we investigated the consequence of Reelin deficiency on synapse formation in adult heterozygous reeler mice using imaging and biochemical approaches. Using a reeler colony that expresses yellow fluorescent protein in selected neurons, we analyzed spine density in hippocampal area CA1 by confocal microscopy and found modest abnormalities in heterozygous reeler mice. However, biochemical analysis of synaptic composition revealed specific postsynaptic defects in scaffolding proteins, neurotransmitter receptors, and signaling proteins. Using whole brain homogenates and purified pre- and postsynaptic fractions, we found that the defects were localized to the postsynaptic compartment of heterozygous reeler synapses. Decreased levels of postsynaptic density-95 (PSD-95), the N-methyl d-aspartate (NMDA) receptor subunits NR2A and NR2B, and the phosphatase PTEN were found specifically in the postsynaptic density fraction obtained from these mice. Furthermore, we found that PSD-95, NR2A, and PTEN interact with each other at the synapse. Finally, we show that levels of NR2A are reduced in conditional Pten knock out mice, demonstrating that the PTEN phosphatase regulates NMDA receptor expression at the synapse in vivo. These studies may provide insights into the etiology of cognitive disorders associated with deficiencies in Reelin signaling and PTEN dysfunction. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Neurons with similar functions including neuronal connectivity and gene expression form discrete condensed structures within the vertebrate brain. This is exemplified within the circuitry formed by the cortical layers and the neuronal nuclei. It is well known that the Reelin protein is required for development of these neuronal structures in rodents and human, but the function of Reelin remains controversial. In this report, we used “layer-specific markers” of the cerebral cortex to carry out detailed observations of spatial distribution of the neuronal subpopulations in the brain of the Reelin deficient mouse, reeler. We observed a spatially dispersed expression of the markers in the reeler cerebral cortex. These markers are expressed also in other laminated and non-laminated structures of brain, in which we observed similar abnormal gene expression. Our observations suggest that neurons within the brain structures (such as the layers and the nuclei), which normally exhibit condensed distribution of marker expressions, loosen their segregation or scatter by a lack of Reelin. [ABSTRACT FROM AUTHOR]

Abstract: The Reelin signaling pathway controls radial neuronal migration and maturation in the developing brain. The platelet activating factor (PAF) acetyl hydrolase 1b (Pafah1b) complex is also involved in multiple aspects of brain development. We previously showed that the Reelin pathway and the Pafah1b complex interact genetically and biochemically. Lis1, the regulatory subunit of Pafah1b interacts with phosphoDab1, an essential mediator of Reelin signaling. Compound mutants carrying mutations in both, the Reelin pathway and Lis1 exhibit hydrocephalus, a phenotype that is suppressed by mutations in the gene encoding the Alpha2 subunit of Pafah1b. This subunit, like the Alpha1 catalytic subunit of Pafah1b also binds the Reelin receptor VLDLR. Here we investigated the molecular interactions of the Pafah1b catalytic subunits with Dab1. We found that Alpha2 coprecipitates with Dab1 from brain extracts of normal and reeler mutant mice lacking Reelin, and from cell-free extracts containing normal or a phosphorylation mutant form of Dab1, suggesting that Dab1 phosphorylation is not necessary for binding to Alpha2. This interaction is specific for Alpha2 and not Alpha1, and depends on a unique tyrosine residue of Alpha2. Biochemical assays using mutant mice lacking Alpha2 further demonstrated that this subunit is not required for Reelin-induced Dab1 phosphorylation. However, increasing amounts of Alpha2 in a cell-free system disrupted the formation of Dab1–Lis1 complexes without affecting the association of Dab1 with VLDLR. Our data suggest that the Alpha2 subunit may play a modulatory role in the formation of protein complexes that affect brain development and hydrocephalus. [Copyright &y& Elsevier]

The reeler mouse is one of the most famous spontaneously occurring mutants in the research field of neuroscience, and this mutant has been used as a model animal to understand mammalian brain development. The classical observations emphasized that laminar structures of the reeler brain are highly disrupted. Molecular cloning of Reelin, the gene responsible for reeler mutant provided insights into biochemistry of Reelin signal, and some models had been proposed to explain the function of Reelin signal in brain development. However, recent reports of reeler found that non-laminated structures in the central nervous system are also affected by the mutation, making function of Reelin signal more controversial. In this review, we summarized reported morphological and histological abnormalities throughout the central nervous system of the reeler comparing to those of the normal mouse. Based on this overview of the reeler abnormalities, we discuss possible function of Reelin signal in the neuronal migration and other morphological events in mouse development. [ABSTRACT FROM AUTHOR]

The development of distinct cellular layers and precise synaptic circuits is essential for the formation of well functioning cortical structures in the mammalian brain. The extracellular protein Reelin, through the activation of a core signaling pathway, including the receptors ApoER2 and VLDLR (very low density lipoprotein receptor) and the adapter protein Dab1 (Disabled-1), controls the positioning of radially migrating principal neurons, promotes the extension of dendritic processes in immature forebrain neurons, and affects synaptic transmission. Here we report for the first time that the Reelin signaling pathway promotes the development of postsynaptic structures such as dendritic spines in hippocampal pyramidal neurons. Our data underscore the importance of Reelin as a factor that promotes the maturation of target neuronal populations and the development of excitatory circuits in the postnatal hippocampus. These findings may have implications for understanding the origin of cognitive disorders associated with Reelin deficiency. [ABSTRACT FROM AUTHOR]

Abstract: Reelin, an extracellular protein that signals through the Dab1 adapter protein, and Lis1 regulate neuronal migration and cellular layer formation in the brain. Loss of Reelin and reduction in Lis1 activity in mice or humans results in the disorganization of cortical structures. Lis1, the product of the Pafah1b1 gene associates with Alpha1 (the product of the Pafah1b3 gene) and Alpha2 (the product of the Pafah1b2 gene) to form the Pafah1b heterotrimeric complex. This complex interacts biochemically and genetically with the Reelin pathway, however, the role of Alpha1 and Alpha2 in brain development is poorly understood. We previously demonstrated that compound mutations of Pafah1b1 with genes in Reelin pathway result in layering defects and the appearance of hydrocephalus in double mutant mice. Here, we generate triple mouse mutants to investigate the effect of individual Pafah1b Alpha subunits on cellular layer formation and hydrocephalus. We found that Pafah1b3 mutations exacerbate the layering defects, whereas Pafah1b2 mutations strongly suppress the hydrocephalus phenotype of compound mutant mice. The data indicate that the two Pafah1b Alpha subunits have profoundly different effects on brain development and interact in a significantly different manner with the Reelin signaling pathway. [Copyright &y& Elsevier]

The reeler gene encodes Reelin, a secreted glycoprotein that binds to the very-low-density lipoprotein receptor (Vldlr) and apolipoprotein E receptor 2 (Apoer 2), and induces Src- and Fyn-mediated tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This Reelin–Dab1 signaling pathway regulates neuronal positioning during development. A second Reelin pathway acts through Apoer 2–exon 19 to modulate synaptic plasticity in adult mice. We recently reported positioning errors in reeler dorsal horn laminae I–II and V, and the lateral spinal nucleus. Behavioral correlates of these positioning errors include a decreased mechanical and increased thermal sensitivity in reeler mice. Here we examined mice with deletions or modifications of both the Reelin–Dab1 signaling pathway and the Reelin–Apoer 2–exon 19 pathway on a Vldlr-deficient background. We detected reeler-like dorsal horn positioning errors only in Dab1 mutant and Apoer 2/Vldlr double mutant mice. Although Dab1 mutants, like reeler, showed decreased mechanical and increased thermal sensitivity, neither the single Vldlr or Apoer 2 knockouts, nor the Apoer 2–exon 19 mutants differed in their acute pain sensitivity from controls. However, despite the dramatic alterations in acute ‘pain’ processing in reeler and Dab1 mutants, the exacerbation of pain processing after tissue injury (hindpaw carrageenan injection) was preserved. Finally, we recapitulated the reeler dorsal horn positioning errors by inhibiting Dab1 phosphorylation in organotypic cultures. We conclude that the Reelin–Dab1 pathway differentially contributes to acute and persistent pain, and that the plasticity associated with the Reelin–Apoer 2–exon 19 pathway is distinct from that which contributes to injury-induced enhancement of ‘pain’ processing. [ABSTRACT FROM AUTHOR]

To clarify whether reelin signaling is involved in dopaminergic neurotransmission in the adult mouse brain, we investigated dopamine function in mice lacking reelin (reeler). We found that methamphetamine-induced locomotor activity is significantly attenuated in reeler mice. To elucidate the mechanism of this phenomenon, we first investigated presynaptic dopamine release; however, there were no significant differences in wildtype, heterozygous reeler and homozygous reeler mice. Next, we examined the locomotor response to intra-accumbens injection of dopamine D1 and D2 receptor agonists, and found that lack of reelin signaling results in decreases in both D1 and D2 receptor-mediated dopaminergic functions. In addition, we measured dopamine receptor binding in the striatum, and found that both D1 and D2 classes of dopamine receptors are reduced in reeler mice. Furthermore, we found that the phosphorylation levels of DARPP-32 are also changed by lack of reelin signaling. Finally, to distinguish between a developmental role of reelin or an acute role of reelin in adult mouse, we intraventricularly infused CR-50, a monoclonal antibody against reelin. Interestingly, infusion of CR-50 also significantly reduced methamphetamine-induced hyperlocomotion in wildtype mice, showing that reelin has an acute role in the dopaminergic system. These results indicate that reelin signaling plays a pivotal role in the dopaminergic system in adult mice, especially in postsynaptic levels. [ABSTRACT FROM AUTHOR]

Abstract: The causative gene for the reeler mouse is reelin which encodes Reelin protein, an extracellular molecule. In the present study, we have examined the cytoarchitecture, myeloarchitecture, and afferent/efferent systems of the superior colliculus (SC) of the reeler mouse. In the reeler, the laminar structures of the superficial three layers of the SC were disorganized and intermingled into a single layer, i.e., the superficial fused layer (SuF), as previously reported in the reelin-deficient SRK rat (Sakakibara et al., Develop. Brain Res. 141:1–13). Next, we have investigated the course and terminals of visual corticotectal and retinotectal projections with an injection of biocytin into the visual cortex or an injection of cholera toxin subunit B into the retina, respectively. In the reeler, anterogradely labeled visual corticotectal and retinotectal fibers took an aberrant course within the SuF, resulting in abnormal myeloarchitecture of the superficial SC of the reeler. Retrograde labeling of tectospinal tract neurons could not show any differences between the normal and reeler mice, suggesting that the deep layers of the reeler SC are cytoarchitectually normal. In situ hybridization and immunohistochemical studies have shown that reelin mRNA and Reelin protein were both recognized in the normal SC. These results suggest that Reelin protein plays some roles in histogenesis of the superficial layers of the SC. [Copyright &y& Elsevier]

Unipolar brush cells (UBCs) are glutamatergic interneurons in the cerebellar cortex and dorsal cochlear nucleus. We studied the development of UBCs, using transcription factor Tbr2/Eomes as a marker for UBCs and their progenitors in embryonic and postnatal mouse cerebellum. Tbr2+ UBCs appeared to migrate out of the upper rhombic lip via two cellular streams: a dorsal pathway into developing cerebellar white matter, where the migrating cells dispersed widely before entering the internal granular layer, and a rostral pathway alongthe cerebellar ventricular zonetowardthe brainstem. Ablation ofthe rhombic lipin organotypic slice cultures substantially reduced the production of Tbr2+ UBCs. In coculture experiments, Tbr2+ UBCs migrated from rhombic lip explants directly into the developing white matter of adjacent cerebellar slices. The origin of Tbr2+ UBCs was confirmed by colocalization with β-galactosidase expressed from the Math1 locus, a molecular marker of rhombic lip lineages. Moreover, the production of Tbr2+ UBCs was Math1 dependent, as Tbr2+ UBCs were severely reduced in Math1-null cerebellum. In reeler mutant mice, Tbr2+ UBCs accumulated near the rhombic lip, consistent with impaired migration through developing white matter. Our results suggest that UBCs arise from the rhombic lip and migrate via novel pathways to their final destinations in the cerebellum and dorsal cochlear nucleus. Our findings support a model of cerebellar neurogenesis, in which glutamatergic and GABAergic neurons are produced from separate progenitor pools located mainly in the rhombic lip and the cerebellar ventricular zone, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Mutations in reeler, the gene coding for the Reelin protein, result in pronounced motor deficits associated with positioning errors (i.e. ectopic locations) in the cerebral and cerebellar cortices. In this study we provide the first evidence that the reeler mutant also has profound sensory defects. We focused on the dorsal horn of the spinal cord, which receives inputs from small diameter primary afferents and processes information about noxious, painful stimulation. We used immunocytochemistry to map the distribution of Reelin and Disabled-1 (the protein product of the reeler gene, and the intracellular adaptor protein, Dab1, involved in its signaling pathway) in adjacent regions of the developing dorsal horn, from early to late embryonic development. As high levels of Dab1 accumulate in cells that sustain positioning errors in reeler mutants, our findings of increased Dab1 immunoreactivity in reeler laminae I–III, lamina V and the lateral spinal nucleus suggest that there are incorrectly located neurons in the reeler dorsal horn. Subsequently, we identified an aberrant neuronal compaction in reeler lamina I and a reduction of neurons in the lateral spinal nucleus throughout the spinal cord. Additionally, we detected neurokinin-1 receptors expressed by Dab1-labeled neurons in reeler laminae I–III and the lateral spinal nucleus. Consistent with these anatomical abnormalities having functional consequences, we found a significant reduction in mechanical sensitivity and a pronounced thermal hyperalgesia (increased pain sensitivity) in reeler compared with control mice. As the nociceptors in control and reeler dorsal root ganglia are similar, our results indicate that Reelin signaling is an essential contributor to the normal development of central circuits that underlie nociceptive processing and pain. [Copyright &y& Elsevier]

Abstract: Reelin is an extracellular matrix protein which is critical for the positioning of migrating post-mitotic neurons and the laminar organization of several brain structures during development. We investigated the expression and localization of Reelin in the rodent peripheral nerve during postnatal development and following crush injury in the adult stage. As shown with Western blotting, immunocytochemistry and RT-PCR, Schwann cells in the developing peripheral nerve and in primary cultures from neonatal nerves produce and secrete Reelin. While Reelin levels are downregulated in adult stages, they are again induced following sciatic nerve injury. A morphometric analysis of sciatic nerve sections of reeler mice suggests that Reelin is not essential for axonal ensheathment by Schwann cells, however, it influences the caliber of myelinated axons and the absolute number of fibers per unit area. This indicates that Reelin may play a role in peripheral nervous system development and repair by regulating Schwann cell–axon interactions. [Copyright &y& Elsevier]

The deep cerebellar nuclei (DCN) are the main output centers of the cerebellum, but little is known about their development. Using transcription factors as cell type-specific markers, we found that DCN neurons in mice are produced in the rhombic lip and migrate rostrally in a subpial stream to the nuclear transitory zone (NTZ). The rhombic lip-derived cells express transcription factors Pax6, Tbr2, and Tbr1 sequentially as they enter the NTZ. A subset of rhombic lip-derived cells also express reelin, a key regulator of Purkinje cell migrations. In organotypic slice cultures, the rhombic lip was necessary and sufficient to produce cells that migrate in the subpial stream, enter the NTZ, and express Pax6, Tbr2, Tbr1, and reelin. In later stages of development, the subpial stream is replaced by the external granular layer, and the NTZ organizes into distinct DCN nuclei. Tbr1 expression persists to adulthood in a subset of medial DCN projection neurons. In reeler mutant mice, which have a severe cerebellar malformation, rhombic lip-derived cells migrated to the NTZ, despite reelin deficiency. Studies in Tbr1 mutant mice suggested that Tbr1 plays a role in DCN morphogenesis but is not required for reelin expression, glutamatergic differentiation, or the initial formation of efferent axon pathways. Our findings reveal underlying similarities in the transcriptional programs for glutamatergic neuron production in the DCN and the cerebral cortex, and they support a model of cerebellar neurogenesis in which glutamatergic and GABAergic neurons are produced from separate progenitor compartments. [ABSTRACT FROM AUTHOR]

Abstract: Reelin (Reln) is a protein involved in migration of newborn neurons during development. Reln mutations produce the reeler phenotype in mice, which is characterized by a defect in brain lamination, and autosomal recessive lissencephaly in humans. Reln expression persists in adult brain, but little is known about its function. We used reeler mice to investigate the effects of Reln deficiency on neurogenesis and the response to injury in the adult brain. Newborn neurons were decreased in number in the dentate gyrus and rostral migratory stream of reeler, compared to wild-type, mice. This was due, at least in part, to impaired cell migration. In addition, reeler mice showed increased susceptibility to ischemic brain injury. Cerebral infarcts from middle cerebral artery occlusion were larger in reeler than in wild-type mice, and associated neurobehavioral abnormalities were more severe. The brains of reeler mice also showed larger excitotoxic lesions after the intracerebral injection of N-methyl-d-aspartate. Finally, despite the fact that reeler mice had larger cerebral infarcts, the ischemia-induced enhancement of neurogenesis observed in wild-type mice was attenuated. These findings suggest that, in addition to its neurodevelopmental effects, Reln deficiency continues to influence neurogenesis and ischemic neuronal injury in the adult brain. [Copyright &y& Elsevier]

Abstract: Developmental defects in neuronal positioning and synaptic connectivity are commonly found in neurological diseases, and they are believed to underlie many cognitive and affective disorders. Several mouse mutants are currently available that model at least some aspects of human developmental brain disorders. With the identification of the genes mutated in these animals and the study of the cellular basis of the phenotypes, we have taken significant strides toward an understanding of the mechanisms controlling proper brain development and the consequences of their dysfunction. In particular, mouse mutants deficient in the Reelin gene have provided valuable insights into the mechanisms of cortical development. Absence of Reelin expression in the spontaneous mutant mouse reeler leads to extensive defects in neuronal position and dendrite development. In humans, loss of Reelin results in a type of lissencephaly with severe cortical and cerebellar malformation. Genetic and biochemical studies using mouse mutants suggest that the Lis1 protein may participate in the Reelin signaling pathway controlling cortical development. Reduced levels of Reelin are also present in postmortem brains of patients with schizophrenia, suggesting a possible link with this cognitive disorder. The regulation of the Reelin gene may thus provide insights into the mechanisms of this disease. [Copyright &y& Elsevier]

Abstract: The cytoarchitecture of dorsal cochlear nucleus (DCN), characterized by a distinct laminar structure similar to the cerebellar cortex of the normal mouse, is known to be disrupted in the Reelin-deficient mouse, reeler. Here, we have reexamined both the cytoarchitecture and myeloarchitecture of this nucleus and described expression pattern of Reelin protein during perinatal periods. Reelin-immunopositive granule cells were firstly recognized in the external granular layer of the DCN at embryological day 16 (E16). Next, we examined the cytoarchitecture of the DCN of the normal and reeler mice with Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) immunostaining. CaMKIIα-immunoreactive cartwheel cells were laminarly distributed in the layer II of the normal DCN, but scattered throughout the reeler DCN. Injection of retrograde tracer, Fluoro-Gold (FG) into the inferior colliculus of the reeler mouse resulted in that retrogradely labeled neurons in the DCN were radially scattered instead of being confined to a single layer as seen in the normal mouse. To examine whether CaMKIIα-immunopositive cartwheel cells are neurons projecting to the inferior colliculus or not, double labeling with CaMKIIα immunohistochemistry and retrograde labeling with an injection of FG into the inferior colliculus were made, which revealed that CaMKIIα-immunoreactive cartwheel cells do not send axons to the inferior colliculus. The present findings imply that Reelin may have some roles in the formation of laminar structures of the DCN. [Copyright &y& Elsevier]

To explore the functional roles of Reelin in the adult olfactory system, we examined changes in the expression ofreelinmRNA and Reelin protein in the olfactory bulb (OB) of adult mice after a chemical lesion to the olfactory epithelium. Following intranasal irrigation with 2% zinc sulphate solution, animals were perfused at various times between 5 and 40 days post-lesion. The expression ofreelinmRNA in mitral cells in the OB was slightly reduced at 5 days post-lesion, completely abolished by 20 days, but restored almost to the normal level at 40 days post-lesion. Similarly, the expression of Reelin protein in mitral cells of the deafferented OB also recovered, although not to the normal level. No recovery of eitherreelinmRNA or Reelin immunoreactivity was seen in the periglomerular cells and external tufted cells. The expression profile ofreelinmRNA and Reelin protein in the OB coincided with the time course of degeneration and regeneration of olfactory nerves, as indicated by anterograde labeling of olfactory nerves with WGA-HRP. These results suggest that expression ofreelinmRNA in the adult OB is regulated by olfactory inputs. [ABSTRACT FROM AUTHOR]

Abstract: The cerebellum of the reeler mutant mouse has an abnormal organization; its single lobule is composed of a severely hypogranular cortex and a central cerebellar mass (CCM) consisting of Purkinje cell clusters intermixing with the cerebellar nuclei. As such the reeler represents an excellent model in which to examine the effect of the abnormal distribution of cerebellar cells on afferent-target relationships. To this effect we studied the organization of the spinocerebellar and secondary vestibulocerebellar afferent projections in homozygous reeler mice (rl/rl) using anterograde tracing techniques. Spinal cord injections resulted in labeled spinocerebellar mossy fiber rosettes in specific anterior and posterior regions of the cerebellar cortex. Some vestiges of parasagittal organization may be present in the anterior projection area. Within the CCM, labeled fibers appeared to terminate on distinct groups of Purkinje cells. Thus, the spinocerebellar mossy fibers seem to form both normal and heterologous synapses in the reeler cerebellum. Secondary vestibular injections resulted in both retrograde and anterograde labeling. Retrograde labeling was seen in clusters of Purkinje cells and cerebellar nuclear cells; anterograde labeling was distributed in the white matter and in specific regions of the anterior and posterior cortex of the cerebellum. The labeled spinocerebellar and secondary vestibulocerebellar afferents overlapped in the anterior region but in the posterior region the vestibulocerebellar termination area was ventral to the spinocerebellar area. An area devoid of labeled terminals was also observed ventral to the posterior secondary vestibulocerebellar termination field. Using calretinin immunostaining it was determined that this area contains unipolar brush cells, a cell type found primarily in the vestibulocerebellum of normal mice. Our data indicate that despite of the lack of known landmarks (fissures, lobules) the spinocerebellar and vestibulocerebellar afferent projections in the reeler cerebellum do not distribute randomly but have specific target regions, and the position of these regions, relative to each other, appears to be conserved. Two caveats to this were the finding of overlapping terminal fields of these afferents in the anterior region, and a posteroventral region that contains unipolar brush cells yet is devoid of secondary vestibulocerebellar afferents. The distribution of Purkinje cells and cerebellar nuclear cells is not random either; those that give rise to cerebellovestibular efferents form distinct groups within the central cerebellar mass. [Copyright &y& Elsevier]

Transient synapse formation between thalamic axons and subplate neurons is thought to be important in thalamocortical targeting. Shaking rat Kawasaki (SRK), having reversed cortical layering similarly observed in reeler mouse, provides an interesting model system to test this idea. The spatial and temporal pattern of excitation was investigated using optical recording with voltage-sensitive dyes in thalamocortical slice preparations from SRK. At postnatal day 0 (P0), a strong optical response was elicited within the superplate of the SRK in the cell layer corresponding to subplate in wild-type (WT) rats. By P3, this response rapidly descended into deep cortical layers comprised of layer IV cells, as identified with 5-bromo-2'-deoxynridine birthdating at embryonic day 17. During the first 3 postnatal days, both the subplate and cortical plate responses were present, but by P7, the subplate response was abolished. Tracing individual axons in SRK revealed that at P0-P3, a large number of thalamocortical axons reach the superplate, and by PT-P10, the ascending axons develop side branches into the lower or middle cortical layers. Synaptic currents were also demonstrated in WT subplate cells and in SRK superficial cortical cells using whole-cell recording. These currents were elicited monosynaptically, because partial AMPA current blockade did not modify the latencies. These results suggest that the general developmental pattern of synapse formation between thalamic axons and subplate (superplate) neurons in WT and SRK is very similar, and individual thalamic arbors in cortex are considerably remodeled during early postnatal development to find layer IV equivalent neurons. [ABSTRACT FROM AUTHOR]

During development, neurons migrate to the cortex radially from periventricular germinative zones as well as tangentially from ganglionic eminences. The vast majority of cortical neurons settle radially in the cortical plate. Neuronal migration requires an exquisite regulation of leading edge extension, nuclear translocation (nucleokinesis), and retraction of trailing processes. During the past few years, several genes and proteins have been identified that are implicated in neuronal migration. Many have been characterized by reference to known mechanisms of neuronal and non-neuronal cell migration in culture; however, probably the most interesting have been identified by gene inactivation or modification in mice and by positional cloning of brain malformation genes in humans and mice. Although it is impossible to provide a fully integrated view, some patterns clearly emerge and are the subject of this article. Specific emphasis is placed on three aspects: first, the role of the actin treadmill, with cyclic formation of filopodial and lamellipodial extensions, in relation to surface events that occur at the leading edge of radially migrating neurons; second, the regulation of microtubule dynamics, which seems to play a key role in nucleokinesis; and third, the mechanisms by which the extracellular protein Reelin regulates neuronal positioning at the end of migration. [ABSTRACT FROM AUTHOR]

During brain development, neurons migrate great distances from proliferative zones to generate the cortical gray matter. A series of studies has identified genes that are critical for migration and targeting of neurons to specific brain regions. These genes encode three basic groups of proteins and produce three distinct phenotypes. The first group encodes cytoskeletal molecules and produces graded and dosage-dependent effects, with a significant amount of functional redundancy. This group also appears to play important roles during the initiation and ongoing progression of neuronal movement. The second group encodes signaling molecules for which homozygous mutations lead to an inverted cortex. In addition, this group is responsible for movement of neurons through anatomic boundaries to specific cortical layers. The third group encodes enzymatic regulators of glycosylation and appears to delineate where neuronal migration will arrest. There is significant cross-talk among these different groups of molecules, suggesting possible points of pathway convergence. [ABSTRACT FROM AUTHOR]

Reelin is an extracellular matrix protein, which plays a crucial role for the formation of laminated and nonlaminated structures in the central nervous system. To elucidate its roles in the postnatal brain, in the present study, we raised a polyclonal antibody specific for rat Reelin, and investigated Reelin-expressing neurons in the rat brain during the postnatal periods in detail. We found that some Reelin-expressing cells existed in the anterior commissure and corpus callosum. These Reelin-expressing cells were also immunostained with the antibody specific for neurons, but not immunostained with the antibodies specific for astrocytes nor oligodendrocytes, suggesting that these Reelin-expressing cells in the white matter are neurons. They are also immunostained with anti-GAD67 antibody, indicating that Reelin-expressing cells in the commissure systems are GABAergic neurons. Reelin-expressing neurons in the anterior commissure had many conspicuous varicosities on their dendritic arbors and mimic to the interfascicular neurons. These results suggest that Reelin may participate in the regulatory mechanism of neuronal activities through the commissure structure during the postnatal periods. [Copyright &y& Elsevier]

The Reelin signaling pathway controls neuronal positioning during mammalian brain development by binding to the very low density lipoprotein receptor and apolipoprotein E receptor-2, and signaling through the intracellular adapter protein Disabled-1 (Dab1). To identify new components in the Reelin signaling pathway, we used a yeast two-hybrid screen to select Dab1-interacting proteins. Here, we report the characterization of a new mouse Dab1-interacting protein that is orthologous to rat Dab2IP, a Ras-GTPase activating protein previously shown to bind to Dab2/DOC. The interaction of Dab1 and Dab2IP was confirmed in biochemical assays and by co-immunoprecipitation from brain lysates. The site of interaction between Dab1 and Dab2IP was narrowed to the Dab1–PTB domain and the NPxY motif in Dab2IP. The deduced amino acid sequence of mouse Dab2IP encompasses 1208 residues containing several protein interaction motifs as well as a Ras-like GAP-related domain. Northern blot analysis revealed at least two isoforms of Dab2IP mRNA in the brain, both of which exhibited increased expression during development. In situ hybridization analyses indicated that Dab2IP mRNA is diffusely expressed throughout the developing and the adult brain. Using a polyclonal antiserum specific for Dab2IP, we observed protein expression in the soma and processes of neurons in a variety of brain structures, including the developing cerebral cortex. Our findings suggest that Dab2IP may function as a downstream effector in the Reelin signaling pathway that influences Ras signaling during brain development. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

During mammalian brain development, neurons are generated along the ventricle, migrate radially, and become aligned in defined patterns. These precise patterns of neuronal alignment are regulated by an extracellular matrix protein Reelin, and binding of Reelin to its receptors induces tyrosine phosphorylation of the intracellular adaptor protein disabled 1 (Dab1). We recently reported that Reelin molecules assemble to form a homomeric protein complex. Although the number of molecules in the full-length complex is unknown, recombinant N-terminal fragments, which contain the epitope for the function-blocking CR-50 antibody, assembled to form a complex of more than 40 monomers. When the N-terminus was deleted from Reelin, the truncated protein did not form a stable complex. To further characterize the Reelin assembly, we performed biochemical analysis of the full-length Reelin assembly in this study. Here, we report that a full-length Reelin forms a disulfide-linked homodimer. A chemical crosslinking experiment on secreted Reelin confirmed that only dimers are formed by the full-length protein. However, interestingly, chemical crosslinking of the N-terminus-truncated Reelin resulted in the formation of larger complexes, in addition to dimers, suggesting that the tertiary structure required for the proper and stable assembly/dimerization was altered by the truncation. The truncated protein did not induce efficient tyrosine phosphorylation of Dab1, although it bound well to the receptors. These findings demonstrate the functional importance of the N-terminal region of Reelin for proper dimerization and signaling. Proper but not simple extracellular crosslinking of the receptors by these dimers may be important for Reelin signaling to occur. [Copyright &y& Elsevier]

Neuronal migration disorders are a category of developmental brain disorders leading to cortical dysplasia. This group of disorders is characterized by defective movement of neurons from the place of origin along the lining of the lateral ventricle, to the eventual place of residence in the correct laminar position within the cerebral cortex. As a result of defective migration, affected individuals typically display mental retardation and epilepsy. Although patients with the more severe forms of these disorders often present during infancy, patients may present at any age from newborn to adulthood. The migration defect may be generalized or focal, and may be disturbed at any of several stages, leading to several distinct radiographical and clinical presentations. The human phenotypes suggests that there are at least four distinct and clinically-important steps in cortical neuronal migration, and the identification of the responsible genes suggests that multiple cellular processes are critical for correct neuronal positioning. MRDD Research Reviews 2001;7:167–171. © 2001 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

The ontogeny of cell adhesion molecule L1 in cerebellum was quantitatively assessed in weaver and reeler mutant mice and in heterozygous litter-mate controls. In the latter the concentration and the amount of L1 both increased from the first postnatal week to become maximum at the second. In contrast, in the weaver and reeler neurologic mutant mice, L1 decreased steadily. The L1 concentration and the amount of L1 was lower in the cerebellum of homozygous mutant mice than in litter-mate controls. The findings are consistent with L1 being a component of axonal plasma membranes. However, no evidence was found of any direct effect of the wv and rl phenotypes on L1 expression. [ABSTRACT FROM AUTHOR]

The authors reflect on Reelin-related neurological disorders and its animal models. They mention the mutation in the Reelin gene of a mouse induced by the neurological phenotype, reeler, which showed neural malpositions, hippocampal cellular layer dyslamination and cerebellar hypoplasia. An overview of its effect on synapse formation in the brain is also presented.