Proteolytic cleavage of the ectodomain of the L1 CAM family member Tractin

Tractin is a member of the L1 family of cell adhesion molecules in leech. Immunoblot analysis suggests that Tractin is constitutively cleaved in vivo at a proteolytic site with the sequence RKRRSR. This sequence conforms to the consensus sequence for cleavage by members of the furin family of convertases, and this proteolytic site is shared by a majority of other L1 family members. We provide evidence with furin-specific inhibitor experiments, by site-specific mutagenesis of Tractin constructs expressed in S2 cells, as well as by Tractin expression in furin-deficient LoVo cells that a furin convertase is the likely protease mediating this processing. Cross-immunoprecipitations with Tractin domain-specific antibodies suggest that the resulting NH(2)- and COOH-terminal cleavage fragments interact with each other and that this interaction provides a means for the NH(2)-terminal fragment to be tethered to the membrane. Furthermore, in S2 cell aggregation assays we show that the NH(2)-terminal fragment is necessary for homophilic adhesion and that cells expressing only the transmembrane COOH-terminal fragment are non-adhesive. However, tethering of exogeneously provided Tractin NH(2)-terminal fragment to S2 cells expressing only the COOH-terminal fragment can functionally restore the adhesive properties of Tractin.

The axonal chemorepellant semaphorin 3A is increased in the cerebellum in schizophrenia and may contribute to its synaptic pathology

The neuropathological features of schizophrenia are suggestive of a developmentally induced impairment of synaptic connectivity. Semaphorin 3A (sema3A) might contribute to this process because it is a secreted chemorepellant which regulates axonal guidance. We have investigated sema3A in the cerebellum (an area in which expression persists in adulthood), and measured its abundance in 16 patients with schizophrenia and 16 controls. In adults, sema3A was predominantly localized to the inner part of the molecular layer neuropil, whereas infants and rats showed greater labelling of Purkinje cell bodies. Sema3A was increased in schizophrenia, as shown by enzyme-linked immunosorbent assay (+28%; P<0.05) and immunohistochemistry (+45%; P<0.01). We also measured reelin mRNA, since reelin is involved in related developmental processes and is decreased in other brain regions in schizophrenia. Reelin mRNA showed a trend reduction in the subjects with schizophrenia (-26%; P=0.07) and, notably, was negatively correlated with sema3A. Sema3A also correlated negatively with synaptophysin and complexin II mRNAs. The results show that sema3A is elevated in schizophrenia, and is associated with downregulation of genes involved in synaptic formation and maintenance. In this respect, sema3A appears to contribute to the synaptic pathology of schizophrenia, perhaps via ongoing effects of persistent sema3A elevation on synaptic plasticity. The findings are consistent with an early neurodevelopmental origin for the disorder, and the reciprocal changes in sema3A and reelin may be indicative of a pathogenic mechanism that affects the balance between trophic and inhibitory factors regulating synaptogenesis.

Disabled-1 mRNA and protein expression in developing human cortex

Disabled-1 (Dab1) forms part of the Reelin-Dab1 signalling pathway that controls neuronal positioning during brain development; Dab1 deficiency gives rise to a reeler-like inversion of cortical layers. To establish a timetable of Dab1 expression in developing human brain, Dab1 mRNA and protein expression were studied in prenatal human cortex. The earliest Dab1 signal was detected at 7 gestational weeks (GW), the stage of transition from preplate to cortical plate, suggesting a role of the Reelin-Dab1 signalling pathway in preplate partition. From 12 to 20 GW, the period of maximum cortical migration, Dab1 expression was prominent in the upper tiers of the cortical plate, to decline after midgestation. Radially orientated apical dendrites of Dab1-expressing neurons indicated a predominant pyramidal phenotype. Pyramidal cells in hippocampus and entorhinal cortex displayed a more protracted time of Dab1 expression compared to neocortex. In addition, at later stages (18-25 GW), Dab1 was also expressed in large neurons scattered throughout intermediate zone and subplate. From 14 to 22 GW, particularly high levels of Dab1 mRNA and protein were observed in cells of the ventricular/subventricular zone displaying the morphology of radial glia. The partial colocalization of vimentin and Dab1 in cells of the ventricular zone supported a radial glia phenotype. The concentration of Dab1 protein in ventricular endfeet and initial portions of radial processes of ventricular-zone cells points to a possible involvement of Dab1 in neurogenesis. Furthermore, a subset of Cajal-Retzius cells in the marginal zone colocalized Dab1 and Reelin, and may thus represent a novel target of the Reelin-Dab1 signalling pathway.

Interspecies comparison of a gene pair with partially redundant function: the rst and kirre genes in D. virilis and D. melanogaster

The D. melanogaster rst and kirre genes encode two highly related immunoglobulin-like cell adhesion molecules that function redundantly during embryonic muscle development. The two genes appear to be derived from a common ancestor by gene duplication. Gene duplications have been proposed to be of major evolutionary significance since duplicated redundant sequences can accumulate mutations without detrimental effects for the organism and leave the duplicated genes free to assume novel functions. To address the issue of conservation of the duplicated sequences and their putative redundancy, as well as to identify putative functional divergence of the paralogs during drosophilid evolution, we performed an interspecies comparison of the rst and kirre genes from D. virilis and D. melanogaster. The D. virilis genome contains orthologues of both rst and kirre and hence the duplication took place before the split of the two lineages and has subsequently been conserved. However, whilst the Rst orthologues show a high degree of sequence similarity, this similarity is lower in Kirre orthologues. Especially the intracellular domains of D. virilis and D. melanogaster Kirre sequences are highly divergent: the D. virilis kirre gene lacks the 3'-most exon present in D. melanogaster, which contains motifs conserved between kirre and rst in D. melanogaster. Hence, while each of the two genes is highly conserved at the level of its exon-intron organization, the selection forces acting on the rst and kirre coding sequences are different. These findings are discussed in the light of general evolutionary mechanisms.

Apolipoprotein E and Reelin ligands modulate tau phosphorylation through an apolipoprotein E receptor/disabled-1/glycogen synthase kinase-3beta cascade

Neurofibrillary tangles comprised of highly phosphorylated tau proteins are a key component of Alzheimer's disease pathology. Mice lacking Reelin (Reln), double-knockouts lacking the VLDL receptor (VLDLR) and ApoE receptor2 (ApoER2), and mice lacking disabled-1 (Dab1) display increased levels of phosphorylated tau. Because Reln binds to recombinant ApoE receptors, assembly of a Reln/ApoE-receptor/Dab1 (RAD) complex may initiate a signal transduction cascade that controls tau phosphorylation. Conversely, disruption of this RAD complex may increase tau phosphorylation and lead to neurodegeneration. To substantiate this concept, we mated Reln-deficient mice to ApoE-deficient mice and found that in the absence of Reln, tau phosphorylation increased as the amount of ApoE decreased. Paralleling the change in tau phosphorylation levels, we found that GSK-3beta activity increased in Reln-deficient mice and further increased in mice lacking both Reln and ApoE. CDK-5 activity was similar in mice lacking Reln, ApoE, or both. GSK-3beta and CDK-5 activity increased in Dab1-deficient mice, independent of ApoE levels. Further supporting the idea that increased tau phosphorylation results primarily from increased kinase activity, the activity of two phosphatases was similar in all conditions tested. These data support a novel, ligand-mediated signal transduction cascade--initiated by the assembly of a RAD complex that suppresses kinase activity and controls tau phosphorylation.

Reelin activates SRC family tyrosine kinases in neurons

BACKGROUND

Reelin is a large signaling molecule that regulates the positioning of neurons in the mammalian brain. Transmission of the Reelin signal to migrating embryonic neurons requires binding to the very-low-density lipoprotein receptor (VLDLR) and the apolipoprotein E receptor-2 (apoER2). This induces tyrosine phosphorylation of the adaptor protein Disabled-1 (Dab1), which interacts with a shared sequence motif in the cytoplasmic tails of both receptors. However, the kinases that mediate Dab1 tyrosine phosphorylation and the intracellular pathways that are triggered by this event remain unknown.

RESULTS

We show that Reelin activates members of the Src family of non-receptor tyrosine kinases (SFKs) and that this activation is dependent on the Reelin receptors apoER2 and VLDLR and the adaptor protein Dab1. Dab1 is tyrosine phosphorylated by SFKs, and the kinases themselves can be further activated by phosphorylated Dab1. Increased Dab1 protein expression in fyn-deficient mice implies a response to impaired Reelin signaling that is also observed in mice lacking Reelin or its receptors. However, fyn deficiency alone does not compound the neuronal positioning defect of vldlr- or apoer2-deficient mice, and this finding suggests functional compensation by other SFKs.

CONCLUSIONS

Our results show that Dab1 is a physiological substrate as well as an activator of SFKs in neurons. Based on genetic evidence gained from multiple strains of mutant mice with defects in Reelin signaling, we conclude that activation of SFKs is a normal part of the cellular Reelin response.

Transgenic mice expressing F3/contactin from the TAG-1 promoter exhibit developmentally regulated changes in the differentiation of cerebellar neurons

F3/contactin (CNTN1) and TAG-1 (CNTN2) are closely related axonal glycoproteins that are differentially regulated during development. In the cerebellar cortex TAG-1 is expressed first as granule cell progenitors differentiate in the premigratory zone of the external germinal layer. However, as these cells begin radial migration, TAG-1 is replaced by F3/contactin. To address the significance of this differential regulation, we have generated transgenic mice in which F3/contactin expression is driven by TAG-1 gene regulatory sequences, which results in premature expression of F3/contactin in granule cells. These animals (TAG/F3 mice) display a developmentally regulated cerebellar phenotype in which the size of the cerebellum is markedly reduced during the first two postnatal weeks but subsequently recovers. This is due in part to a reduction in the number of granule cells, most evident in the external germinal layer at postnatal day 3 and in the inner granular layer between postnatal days 8 and 11. The reduction in granule cell number is accompanied by a decrease in precursor granule cell proliferation at postnatal day 3, followed by an increase in the number of cycling cells at postnatal day 8. In the same developmental window the size of the molecular layer is markedly reduced and Purkinje cell dendrites fail to elaborate normally. These data are consistent with a model in which deployment of F3/contactin on granule cells affects proliferation and differentiation of these neurons as well as the differentiation of their synaptic partners, the Purkinje cells. Together, these findings indicate that precise spatio-temporal regulation of TAG-1 and F3/contactin expression is critical for normal cerebellar morphogenesis.

The Neurobiological Context of Autism

Autistic disorder (AD) is a complex neuropsychiatric disorder of neurodevelopmental origin, where multiple genetic and environmental factors may interact, resulting in a clinical continuum. The genetic component is best described by a multilocus model that takes into account epistatic interactions between several susceptibility genes. In the past ten years enormous progress has been made in identifying chromosomal regions in linkage with AD, but moving from chromosomal regions to candidate genes has proven to be tremendously difficult. Neuroanatomical findings point to early dysgenetic events taking place in the cerebral cortex, cerebellum, and brainstem. At the cellular level, disease mechanisms may include altered cell migration, increased cell proliferation, decreased cell death, or altered synapse elimination. Neurochemical findings in AD point to involvement of multiple neurotransmitter systems. The serotoninergic system has been intensively investigated in AD, but other neurotransmitter systems (e.g., the GABAergic and the cholinergic system) are also coming under closer scrutiny. The role of environmental factors is still poorly characterized. It is not clear yet whether environmental factors act merely as precipitating agents, always requiring an underlying genetic liability, or whether they represent an essential component of a pathogenetic process where genetic liability alone does not lead to the full-blown autism phenotype. A third potential player in the pathogenesis of autism, in addition to genetic and environmental factors, is developmental variability due to "random" factors, e.g. small fluctuations of gene expression and complex, non-deterministic interactions between genes during brain development. These considerations suggest that a non-deterministic conceptual framework is highly appropriate for autism research.

The thrombospondin type 1 repeat (TSR) and neuronal differentiation: roles of SCO-spondin oligopeptides on neuronal cell types and cell lines

SCO-spondin is a large glycoprotein secreted by ependymal cells of the subcommissural organ. It shares functional domains called thrombospondin type 1 repeats (TSRs) with a number of developmental proteins expressed in the central nervous system, and involved in axonal pathfinding. Also, SCO-spondin is highly conserved in the chordate phylum and its multiple domain organization is probably a chordate innovation. The putative involvement of SCO-spondin in neuron/glia interaction in the course of development is assessed in various cell culture systems. SCO-spondin interferes with several developmental processes, including neuronal survival, neurite extension, neuronal aggregation, and fasciculation. The TSR motifs, and especially the WSGWSSCSVSCG sequence, are most important in these neuronal responses. Integrins and growth factor receptors may cooperate as integrative signals. We discuss the putative involvement of the subcommissural organ/Reissner's fiber complex in developmental events, as a particular extracellular signaling system.

An epigenetic mouse model for molecular and behavioral neuropathologies related to schizophrenia vulnerability

Reelin and glutamic acid decarboxylase (GAD)67 expressed by cortical gamma-aminobutyric acid-ergic interneurons are down-regulated in schizophrenia. Because epidemiological studies of schizophrenia fail to support candidate gene haploinsufficiency of Mendelian origin, we hypothesize that epigenetic mechanisms (i.e., cytosine hypermethylation of CpG islands present in the promoter of these genes) may be responsible for this down-regulation. Protracted l-methionine (6.6 mmolkg for 15 days, twice a day) treatment in mice elicited in brain an increase of S-adenosyl-homocysteine, the processing product of the methyl donor S-adenosyl-methionine, and a marked decrease of reelin and GAD67 mRNAs in both WT and heterozygous reeler mice. This effect of l-methionine was associated with an increase in the number of methylated cytosines in the CpG island of the reelin promoter region. This effect was not observed for GAD65 or neuronal-specific enolase and was not replicated by glycine doses 2-fold greater than those of l-methionine. Prepulse inhibition of startle declined at a faster rate as the prepulsestartle interval increased in mice receiving l-methionine. Valproic acid (2 mmolkg for 15 days, twice a day) reverted l-methionine-induced down-regulation of reelin and GAD67 in both WT and heterozygous reeler mice, suggesting an epigenetic action through the inhibition of histone deacetylases. The same dose of valproate increased acetylation of histone H3 in mouse brain nearly 4-fold. This epigenetic mouse model may be useful in evaluating drug efficacy on schizophrenia vulnerability. Hence the inhibition of histone deacetylases could represent a pharmacological intervention mitigating epigenetically induced vulnerability to schizophrenia in individuals at risk.

A novel splice variant of the cell adhesion molecule contactin 4 ( CNTN4) is mainly expressed in human brain

Axon-associated cell adhesion molecules (AxCAMs) of the immunoglobulin superfamily play important roles in the formation, maintenance, and plasticity of functional neuronal networks. Contactin4 ( CNTN4, BIG-2) is a member of the TAG-1/F3 subgroup of AxCAMs. We have cloned a novel splice variant of CNTN4, and term it CNTN4A. The complete nucleotide sequence of CNTN4 is also obtained by combining the insert sequences of two clones, which were isolated when screening the human fetal brain cDNA library with CNTN4A as a probe. CNTN4A protein has an N-terminal cleavable signal peptide, two FNIII-like domains, and a glycosyl phosphatidylinositol-anchoring domain. According to the search of the human genome database, CNTN4 was mapped to 3p25-26, a region very close to the breakpoints of the 3p syndrome. Expression analysis of CNTN4A shows that CNTN4A is mainly expressed in brain.

Abnormal distributions of callosal commissural and corticothalamic neurons in the cerebral neocortex of Shaking Rat Kawasaki

Shaking Rat Kawasaki (SRK) is an autosomal recessive mutant rat recognized by unstable gait and tremor and by early death around the time of weaning. We previously reported that corticospinal tract neurons are malpositioned in the motor cortex of the SRK rat [Ikeda and Terashima (1997) J. Comp. Neurol. 383, 370-380]. In the present study, we examined the distribution pattern of callosal commissural (CC) and corticothalamic (CT) neurons of SRK and normal rats with the injection of horseradish peroxidase (HRP) into the contralateral hemisphere or wheat germ agglutinin-conjugated HRP into the ventral lateral thalamic nucleus. The intracortical distribution pattern of retrogradely labeled CC and CT neurons in the motor cortex of SRK rat was abnormal: CC neurons were more deeply situated and CT neurons were more superficially situated in the SRK cortex than the corresponding components in the normal cortex. Most of labeled CC and CT neurons had abnormal dendritic configurations. Statistical analysis revealed that the difference of the mean intracortical position of CC and CT neurons of the SRK was significantly different from the normal counterparts (Student's t-test, P<0.01). Taken together with previous findings, our data demonstrate that the abnormal cytoarchitecture of SRK cortex resembles the reeler cortex.

A secreted soluble form of ApoE receptor 2 acts as a dominant-negative receptor and inhibits Reelin signaling

Specialized neurons throughout the developing central nervous system secrete Reelin, which binds to ApoE receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR), triggering a signal cascade that guides neurons to their correct position. Binding of Reelin to ApoER2 and VLDLR induces phosphorylation of Dab1, which binds to the intracellular domains of both receptors. Due to differential splicing, several isoforms of ApoER2 differing in their ligand-binding and intracellular domains exist. One isoform harbors four binding repeats plus an adjacent short 13 amino acid insertion containing a furin cleavage site. It is not known whether furin processing of this ApoER2 variant actually takes place and, if so, whether the produced fragment is secreted. Here we demonstrate that cleavage of this ApoER2 variant does indeed take place, and that the resulting receptor fragment consisting of the entire ligand-binding domain is secreted as soluble polypeptide. This receptor fragment inhibits Reelin signaling in primary neurons, indicating that it can act in a dominant-negative fashion in the regulation of Reelin signaling during embryonic brain development.

Association analysis of polymorphic CGG repeat in 5' UTR of the reelin and VLDLR genes with schizophrenia

Several lines of evidence suggest a possible role for reelin in the pathogenesis of neurodevelopmental diseases, particularly schizophrenia. Genes encoding reelin and proteins involved in the signal pathway of reelin are thus candidate genes for schizophrenia. We examined the polymorphic CGG repeat in the 5'-untranslated region (UTR) of the reelin gene, which was recently found to be associated with autistic disorder, and the CGG repeat in the 5' UTR region of the very low density protein receptor (VLDLR) gene, which was reported to be associated with sporadic Alzheimer's disease, for allelic association with schizophrenia. The subjects consisted of 150 patients and 150 controls matched for sex, age and ethnicity (Japanese). We found no significant association of schizophrenia with the trinucleotide repeat polymorphism of the reelin or VLDLR genes, suggesting that these polymorphisms do not have a major role in the pathogenesis of the disease.

Expression of the zebrafish recognition molecule F3/F11/contactin in a subset of differentiating neurons is regulated by cofactors associated with LIM domains

We have identified a zebrafish homolog of the F3/F11/contactin (F3) recognition molecule. The gene shares 55% amino acid identity with F3 molecules in other vertebrates. Expression of F3 mRNA is first detectable at 16 h post-fertilization (hpf) in trigeminal and Rohon-Beard neurons. At 18-24 hpf, additional weaker expression is present in discrete cell clusters in the hindbrain, in the anterior lateral line/acoustic ganglion and in spinal motor neurons. Transcription factors of the LIM homeodomain class (LIM-HD) and their associated cofactors CLIM/NLI/Ldb (CLIM) have been implicated in the development of peripheral axons of trigeminal and Rohon-Beard neurons. We demonstrate that ectopic overexpression of a dominant-negative CLIM molecule early during zebrafish development strongly reduces expression of F3 mRNA in these neurons indicating regulation of F3 by the LIM-HD protein network. These results and the spatiotemporal correlation of F3 expression with axonal differentiation in a subset of primary neurons suggest an important role of F3 for axon growth.

Pax6 is required for the normal development of the forebrain axonal connections

The transcription factor PAX6 has been implicated in forebrain patterning,cerebral cortical arealization and in development of thalamocortical connections. Using a Pax6/lacZ knockout mouse, in which the endogenous Pax6 expression is reflected by β-galactosidase activity, we have studied the consequences of the loss of Pax6function on thalamocortical (TCA) and corticofugal axon (CFA) pathfinding during the period of embryonic day (E) 14.5 to E18.5. Carbocyanine dye tracing in Pax6 heterozygotes (Pax6+/-) and Pax6wild-type (Pax6+/+) brains revealed that CFAs and TCAs temporarily arrested their growth at E14.5 at the border of theβ-galactosidase-positive region at the pallial/subpallial boundary(PSPB), before they continued towards their targets. However, in Pax6homozygous (Pax6-/-) embryos, CFAs and TCAs were unable to encounter each other at the PSPB and reach their final targets. Instead of crossing the PSPB, they had the tendency to descend into the ventral pallium in large aberrant fascicles. In addition, cells with a presumptive guide-post function, which are normally situated in the ventral thalamus, internal capsule and hypothalamus, were more dispersed in the hypothalamus and ventral pallium. These pathfinding defects were confirmed by immunohistochemistry for L1 and TAG1, markers of the early axonal connections. The aberrant development of axonal connections in absence of Pax6 function appear to be related to ultrastructural defects of cells along the PSPB, as well as to a failure of axonal guidance molecule expression, including Sema3c and Sema5a.

Brain development and the genetics of brain development

The progress made in the understanding of the genetics of human brain malformations has lead to insight into the formation of brain and into mechanisms of disease affecting brain. It bears upon neurologists and geneticists to recognize the patterns of diseases of brain formation, to properly diagnose such disorders, to assess the recurrence risk of these malformations, and to guide families with appropriate expectations for outcomes. This article may serve as a guide to neurologists in their approach to these disorders. Because this area is one of rapid progress, the clinician is advised to seek more current information that may be available through on-line databases and other sources.

Reelin is a detachment signal in tangential chain-migration during postnatal neurogenesis

During development, Reelin acts on migrating neuronal precursors and controls correct cell positioning in the cortex and other brain structures by a hitherto unidentified mechanism. Here we show that in the postnatal mouse brain, Reelin acts as a detachment signal for chain-migrating interneuron precursors in the olfactory bulb. Neuronal precursors cultured in Matrigel detached from chains and migrated individually in the presence of exogenously added Reelin protein or Reelin-expressing brain tissues. Furthermore, we found that in reeler mutant mice, neuronal precursors accumulated in the olfactory bulb and remained in clusters, indicating that they did not change from tangential chain-migration to radial individual migration. Our data provide direct evidence that Reelin acts as a detachment signal, but not a stop or guidance cue. We propose that Reelin may have comparable functions during development.

Reelin signaling and Cdk5 in the control of neuronal positioning

Neuronal positioning is important for higher brain function because it is the architectural basis of the formation of precise synaptic circuits. Analysis of neurological mutant mice has led to dramatic progress in the identification and characterization of molecules important for neuronal positioning in the developing mammalian brain. Among these molecules, identification of signal pathways mediated by Reelin and Cdk5 kinase has provided a conceptual framework for exploring the molecular mechanisms underlying proper neuronal positioning in the developing mammalian brain. Recent evidence has implicated synergism between Reelin signaling and Cdk5 in contributing to the proper positioning of selective neuronal populations.

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

The extracellular matrix molecule Reelin is required for the correct positioning of neurons during the development of the forebrain. However, the mechanism of Reelin action on neuronal migration is poorly understood. Reelin is assumed to act on neurons directly, but it may also affect the differentiation of glial cells necessary for neuronal migration. Here we show that a regular glial scaffold fails to form in vivo in the dentate gyrus of mice deficient of Reelin or Disabled 1, a neuronal adaptor protein in the Reelin signaling pathway. A subset of these defects is observed in mice that lack beta(1)-class integrins, known to bind Reelin. Moreover, recombinant Reelin induced branching of glial processes in vitro. Our data suggest that Reelin affects glial differentiation via Disabled 1 and beta(1)-class integrin-dependent signaling pathways.

A reeler mutant mouse with a new, spontaneous mutation in the reelin gene

In one of our mouse colonies a reeler-like phenotype appeared spontaneously. The brain histology was identical to the known reeler phenotype. Northern and Western blot analysis and a complementation test showed that the defect is located to the reelin gene. Southern blot and PCR analysis together with information obtained from sequence databases revealed that this defective reelin gene had an approximately 24-kb intragenic deletion comprising exons 13-20.

In Patas monkey, glutamic acid decarboxylase-67 and reelin mRNA coexpression varies in a manner dependent on layers and cortical areas

In nonhuman and human primates, reelin immunoreactivity is expressed consistently in gamma-aminobutyric acid (GABA)-ergic interneurons of the three upper cortical layers (Impagnatiello et al. [1998] Proc. Natl. Acad. Sci. U S A 95:15718-15723; Rodriguez et al. [2000] Proc. Natl. Acad. Sci. U S A 97:3550-3555). To understand in detail the pattern of reelin synthesis in GABAergic interneurons of primate neocortex, a quantitative analysis of reelin and of glutamic acid decarboxylase-67 (GAD(67)) mRNA-positive neurons as well as a quantitative analysis of total neuronal density measured by neuron-specific nuclear protein (NeuN) immunoreactivity was carried out in Patas monkey neocortex (Brodmann's areas 2, 3, 4, 6, 9, 17, 18, and 24). Reelin mRNA is expressed in every cortical area and layer studied, but layer II of each cortical area consistently revealed the largest neuronal population expressing reelin mRNA compared with other layers. The percentages of GAD(67)-positive neurons in each layer of the eight cortical areas were 83-98% in layer I, 55-64% in layer II, 37-49% in layer III, 71-89% in layer IV, 54-68% in layer V, and 71-85% in layer VI. The percentages of GABAergic neurons expressing reelin were 86-100% in layer I, 76-84% in layer II, 52-96% in layer III, 23-33% in layer IV, 33-57% in layer V, and 34-54% in layer VI. These findings suggest that there may be two classes of GABAergic neurons that can be differentiated by their ability to express reelin mRNA and reelin protein. This differentiation may have a functional significance, considering that reelin is secreted into the extracellular matrix, where it plays a putative role in the maturation of newly formed dendritic spines and binds selectively to dendritic shafts and to spine postsynaptic densities and presumably to integrin receptors, including alpha(3) subunits (Rodriguez et al. [2000]).

Neurons tend to stop migration and differentiate along the cortical internal plexiform zones in the Reelin signal-deficient mice

The Reelin molecule plays a fundamental role in corticogenesis. After Reelin binds to its receptors, the Reelin signal is transduced through tyrosine phosphorylation of the intracellular adaptor protein disabled 1 (Dab1). The reelin-gene-deficient mouse, reeler, and Dab1-deficient mouse, yotari, show disrupted positioning of neurons. Several molecules have been identified recently as being involved in Reelin signaling, however, the biological function of Reelin during cortical plate development was still unknown. We observed the migrating behavior of neurons during development in Reelin-signal-deficient mice. To visualize the migrating neurons directly, we introduced green fluorescent protein (GFP)-expression vectors into the ventricular zone with an in utero electroporation system and allowed the embryos to develop in utero until they were analyzed. The result showed that the migrating cells in the mutants were morphologically indistinguishable from those of normal mice. At the stage when the GFP-expressing cells reached the marginal zone near the pial surface and began dendrite formation in normal mice, the GFP-positive cells were found at various deeper positions in the mutant cortex. They had the morphology of migrating cells extending leading processes toward the pial surface. By contrast, in the mutants these cells tended to stop migration along the borders of the internal plexiform zone, the irregular structure consisting mainly of dendrites in the mutant cortex. Postnatally, these neurons began to develop dendrites later than the cells in the normal cortex. During this process, some neurons above the internal plexiform zone extended and developed dendrites in the opposite direction into the internal plexiform zone. These results suggest that the abnormal positioning of neurons in the Reelin-signal-deficient mice is caused, at least in part, by abnormal formation of the internal plexiform zone in the mutant cortex.

Analysis of genes induced in peripheral nerve after axotomy using cDNA microarrays

One of the most striking features of neurons in the mature peripheral nervous system is their ability to survive and to regenerate their axons following axonal injury. To perform a comprehensive survey of the molecular mechanisms that underlie peripheral nerve regeneration, we analyzed a cDNA library derived from the distal stumps of post-injured sciatic nerve which was enriched in non-myelinating Schwann cells using cDNA microarrays. The number of up- and down-regulated genes in the transected sciatic nerve was 370 and 157, respectively, of the 9596 spotted genes. In the up-regulated group, the number of known genes was 216 and the number of expressed sequence tag (EST) sequences was 154. In the down-regulated group, the number of known genes was 103 and that of EST sequences was 54. We obtained several genes that were previously reported to be involved in regeneration of the injured neurons, such as cathepsin D, ninjurin 1, tenascin C, and co-receptor for glial cell line-derived neurotrophic factor family of trophic factors. In addition to unknown genes, there seemed to be a lot of annotated genes whose role in nerve regeneration remains unknown.