Recent progress in understanding the role of Reelin in radial neuronal migration, with specific emphasis on the dentate gyrus

Emerging topics in Reelin function

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

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

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

Migratory defect of mesencephalic dopaminergic neurons in developing reeler mice

Reelin, an extracellular glycoprotein has an important role in the proper migration and positioning of neurons during brain development. Lack of reelin causes not only disorganized lamination of the cerebral and cerebellar cortex but also malpositioning of mesencephalic dopaminergic (mDA) neurons. However, the accurate role of reelin in the migration and positioning of mDA neurons is not fully elucidated. In this study, reelin-deficient reeler mice exhibited a significant loss of mDA neurons in the substantia nigra pars compacta (SNc) and a severe alteration of cell distribution in the retrorubal field (RRF). This abnormality was also found in Dab1-deficinet, yotari mice. Stereological analysis revealed that total number of mDA neurons was not changed compared to wild type, suggesting that the loss of mDA neurons in reeler may not be due to the neurogenesis of mDA neurons. We also found that formation of PSA-NCAM-positive tangential nerve fibers rather than radial glial fibers was greatly reduced in the early developmental stage (E14.5) of reeler. These findings provide direct evidence that the alteration in distribution pattern of mDA neurons in the reeler mesencephalon mainly results from the defect of the lateral migration using tangential fibers as a scaffold.

Cellular form of prion protein inhibits Reelin-mediated shedding of Caspr from the neuronal cell surface to potentiate Caspr-mediated inhibition of neurite outgrowth

Extension of axonal and dendritic processes in the CNS is tightly regulated by outgrowth-promoting and -inhibitory cues to assure precision of synaptic connections. We identify a novel role for contactin-associated protein (Caspr) as an inhibitory cue that reduces neurite outgrowth from CNS neurons. We show that proteolysis of Caspr at the cell surface is regulated by the cellular form of prion protein (PrP), which directly binds to Caspr. PrP inhibits Reelin-mediated shedding of Caspr from the cell surface, thereby increasing surface levels of Caspr and potentiating the inhibitory effect of Caspr on neurite outgrowth. PrP deficiency results in reduced levels of Caspr at the cell surface, enhanced neurite outgrowth in vitro, and more efficient regeneration of axons in vivo following spinal cord injury. Thus, we reveal a previously unrecognized role for Caspr and PrP in inhibitory modulation of neurite outgrowth in CNS neurons, which is counterbalanced by the proteolytic activity of Reelin.

The effect of amygdala kindling on hippocampal neurogenesis coincides with decreased reelin and DISC1 expression in the adult dentate gyrus

Temporal lobe seizures can induce the proliferation and abnormal migration of newly generated dentate granule cells, but little is known about the molecular mechanisms that govern these pathological events. Reelin and DISC1 (disrupted-in-schizophrenia 1) are proteins that play a regulatory role in the maturation and integration of new neurons in the developing and adult brain. In this study, we examined whether amygdala kindling results in aberrant neurogenesis and altered expression of reelin and DISC1 in the adult dentate gyrus. Using doublecortin immunohistochemistry, we found that short-term kindling (i.e., 30 electrical stimulations) significantly increased the number of immature neurons in the dentate subgranular zone (SGZ), whereas long-term kindling (i.e., 99 electrical stimulations) did not. However, doublecortin-labeled neurons in long-term kindled rats showed greater dendritic complexity than they did in short-term kindled or control rats. We also found that long-term kindling decreased the number of reelin-positive cells and decreased DISC1 expression in the dentate granule cell layer and subgranular zone. Interestingly, kindling-induced changes in reelin and DISC1 expression coincided with the appearance of ectopically located Prox1-labeled granule cells in the hilus. These effects occurred independently of alterations in granule cell layer length, dentate volume, or the number of hilar neurons. Taken together, these findings suggest a novel role for DISC1 in the pathophysiology of temporal lobe epilepsy and further suggest that changes in reelin and DISC1 expression may contribute to aberrant neurogenesis in the kindling model.

Behavioral and neurobiological consequences of prolonged glucocorticoid exposure in rats: relevance to depression

Stress is a critical environmental trigger for the development of clinical depression, yet little is known about the specific neurobiological mechanisms by which stress influences the development of depressive symptomatology. Animal models provide an efficient way to study the etiology of human disorders such as depression, and a number of preclinical models have been developed to assess the link between stress, glucocorticoids, and depressive behavior. These mode ls typically make use of repeated exposure to physical or psychological stressors in rodents or other small laboratory animals. This review focuses primarily on a recently developed preclinical model of depression that uses exogenous administration of the stress hormone corticosterone (CORT) in rodents instead of exposure to physical or psychological stressors. Repeated CORT administration in rats or mice produces reliable behavioral and neurobiological alterations that parallel many of the core symptoms and neurobiological changes associated with human depression. This provides an opportunity to study behavior and neurobiology in the same animal, so that the neurobiological factors that underlie specific symptoms can be identified. Taken together, these findings suggest that exogenous CORT administration is a useful method for studying the relationship between stress, glucocorticoids, and depression. Further study with this model may provide important new data regarding the neurobiological bases of depression.

Reelin expression during embryonic development of the pig brain

Background

Reelin is an extracellular glycoprotein of crucial importance in the developmental organisation of neurons in the mammalian cerebral cortex and other laminated brain regions. The pig possesses a gyrencephalic brain that bears resemblance to the human brain. In order to establish an animal model for neuronal migration disorders in the pig, we have studied the expression pattern and structure of Reelin during pig brain development.

Results

We determined the sequence of pig Reelin mRNA and protein and identified a high degree of homology to human Reelin. A peak in Reelin mRNA and protein expression is present during the period of major neurogenesis and neuronal migration. This resembles observations for human brain development. Immunohistochemical analysis showed the highest expression of Reelin in the Cajal-Reztius cells of the marginal zone, in resemblance with observations for the developing brain in humans and other mammalian species.

Conclusions

We conclude that the pig might serve as an alternative animal model to study Reelin functions and that manipulation of the pig Reelin could allow the establishment of an animal model for human neuronal migration disorders.

Distinctive properties of CXC chemokine receptor 4-expressing Cajal-Retzius cells versus GABAergic interneurons of the postnatal hippocampus

The CXC chemokine receptor 4 (CXCR4) for the chemokine (C-X-C motif) ligand 12/stromal cell-derived factor-1 alpha (CXCL12/SDF-1 alpha) is highly expressed in the postnatal CA1 stratum lacunosum-moleculare. However, both the network events triggered by SDF-1 alpha in this microcircuit and the cellular targets of this chemokine remain virtually unexplored. Here, we have studied SDF-1 alpha-mediated neuromodulation of the stratum lacunosum-moleculare by directly comparing the properties of CXCR4-expressing Cajal-Retzius cells vs. CXCR4-non-expressing interneurons, and by recording the electrophysiological effects caused by application of SDF-1 alpha on either cell type. We demonstrate that SDF-1 alpha dramatically reduces spontaneous firing in Cajal-Retzius cells via hyerpolarization, and that cessation of firing is prevented by the CXCR4-specific antagonist AMD3100. In contrast, no effects on the excitability of interneurons of the same layer were observed following exposure to the chemokine. We also provide evidence that, despite the expression of functional glutamate receptors, Cajal-Retzius cells are integrated in the synaptic network of the stratum lacunosum-moleculare via excitatory GABAergic input. Furthermore, we show that the axons of Cajal-Retzius cells target specifically the stratum lacunosum-moleculare and the dentate gyrus, but lack postsynaptic specializations opposite to their axonal varicosities. These results, taken together with our observation that SDF-1 alpha reduces evoked field responses at the entorhinal cortex-CA1 synapse, suggest that Cajal-Retzius cells produce a diffuse output that may impact information processing of stratum lacunosum-moleculare. We propose that pathological alterations of local levels of SDF-1 alpha or CXCR4 expression may affect the functions of an important hippocampal microcircuit.

[Language disorders in children with morphologic abnormalities of the hippocampus]

PURPOSE

Morphologic abnormalities of the hippocampal formations (MAHF) are more frequently observed in magnetic resonance imaging (MRI). We wished to specify the types of disorders associated with these malformations based on a retrospective case series by studying the language of the children presenting these abnormalities.

PATIENTS AND METHODS

From the data of all the MRIs taken in the neuroradiology ward of our center over 16 months in patients under 18 years of age, we retrospectively selected the children with an MAHF, isolated or associated with other malformations. The MAHFs were defined and described according to criteria of shape or orientation defects of the hippocampal formations. We studied the files of the patients with isolated MAHF again. Those whose clinical presentation was compatible with language assessment were tested in a prospective approach.

RESULTS

Out of 2208 MRIs from 1 January 2007 to 30 April 2008, 96 (4.3%) showed an MAHF, including 61 (64%) boys and 35 (36%) girls, aged from 2 months to 17 years. Eighty-two (85%) had associated abnormalities, mainly including cerebral atrophy, corpus callosum agenesis or defect, and abnormal ventricular frontal horns. Fourteen (15%) had an isolated MAHF: 2 on the left hemisphere, 2 on the right hemisphere, and 10 on both. Of these 14, 9 were compatible with language assessment. From the test results, we divided these children into 2 groups, depending on the type and severity of the impairment. Four had very serious language disorders as part of mental retardation or autistic disorders; 4 others had language disorders predominantly in expression and phonology, with weak to pathological visual memory. This study showed no potential relation between the lateralization of MAHF and language disorders, nor between the existence of epilepsy and the severity of the language disorders. Of these 14 children, 9 had behavior and autism spectrum disorders and 7 were epileptic.

CONCLUSION

Even though language disorders are often part of a larger deficiency presentation, the results we obtained suggest that isolated MAHFs are not only causes of amnestic disorders, but they could also directly underlie language disorders, particularly in expression.

Ceruloplasmin-induced aggregation of P19 neurons involves a serine protease activity and is accompanied by reelin cleavage

The cytoarchitectural organization of the nervous system depends partly on extracellular serine proteases, including reelin. This 400K protein, which also exists as the N-terminally-derived 300K and 180K fragments, acts through binding to the lipoprotein receptors apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor (VLDLR). Ceruloplasmin (CP), a multifunctional protein found in the circulation and also expressed on glial cells, was shown to bind to, and induce aggregation of neurons newly differentiated from P19 embryonic stem cells. This indicated a potential developmental role of CP in neuronal organization, possibly in relation with reelin and other extracellular serine proteases. Therefore, we analysed the effect of cell-impermeant, large spectrum, serine protease inhibitors on CP-induced neuroaggregation and studied reelin expression. Soybean trypsin inhibitor and aprotinin (SBTI+Apro) inhibited CP neuroaggregative action. Undifferentiated and neurally-differentiating cultures secreted the 400K reelin. The 180K fragment was present during and after differentiation whereas the 300K species was barely detectable. However, CP stimulated generation of the 300K in the differentiated neuronal cultures, and SBTI+Apro abolished this CP effect. Time course profiles and function-blocking antibody indicated that neuroaggregation does not depend on the generation of the 300K fragment or on reelin action. CP neuroaggregative action thus involves a pericellular serine protease, different from reelin. On the other hand, the CP stimulation of reelin cleavage is in line with a possible role of CP in nervous system development. Since P19 cells express ApoER2 and VLDLR, they can help understanding relationships existing between CP, reelin and intervening protease(s).

Differential functions of ApoER2 and very low density lipoprotein receptor in Reelin signaling depend on differential sorting of the receptors

ApoER2 and very low density lipoprotein (VLDL) receptor transmit the Reelin signal into target cells of the central nervous system. To a certain extent, both receptors can compensate for each other, and only the loss of both receptors results in the reeler phenotype, which is characterized by a gross defect in the architecture of laminated brain structures. Nevertheless, both receptors also have specific distinct functions, as corroborated by analyses of the subtle phenotypes displayed in mice lacking either ApoER2 or VLDL receptor. The differences in their function(s), however, have not been defined at the cellular level. Here, using a panel of chimeric receptors, we demonstrate that endocytosis of Reelin and the fate of the individual receptors upon stimulation are linked to their specific sorting to raft versus non-raft domains of the plasma membrane. VLDL receptor residing in the non-raft domain endocytoses and destines Reelin for degradation via the clathrin-coated pit/clathrin-coated vesicle/endosome pathway without being degraded to a significant extent. Binding of Reelin to ApoER2, a resident of rafts, leads to the production of specific receptor fragments with specific functions of their own and to degradation of ApoER2 via lysosomes. These features contribute to a receptor-specific fine tuning of the Reelin signal, leading to a novel model that emphasizes negative feedback loops specifically mediated by ApoER2 and VLDL receptor, respectively.

Reelin and apolipoprotein E receptor 2 in the embryonic and mature brain: effects of an evolutionary change in the apoER2 gene

In the mature cerebral cortex of higher vertebrates, neurons are arranged in layers, each layer containing neurons of the same functional class. The cortical layering pattern is laid down during development by migration of young post-mitotic neurons along glial fibres to their correct positions in the cortical plate. The mechanics of whole-cell movement are well understood, but there is still uncertainty as to how a migrating neuron is instructed to leave its glial support when it reaches its destination. An intraneuronal signalling pathway initiated by reelin and containing apolipoprotein E receptor 2 (apoER2) is essential for normal cortical layering, and there is strong evidence that detachment of a migrating neuron is brought about by reelin-dependent downregulation of alpha3 integrin. But there remains the problem of how the reelin signal is switched on at a position in the cortex appropriate for each class of neuron. ApoER2 of placental mammals contains an amino acid sequence that is encoded in a separate exon in the apoER2 gene and is required for normal memory and spatial learning. The separate exon is not present in marsupials, birds or reptiles. The addition of this exon to the evolving apoER2 gene may have contributed to the success of placental mammals.

Reelin stabilizes the actin cytoskeleton of neuronal processes by inducing n-cofilin phosphorylation at serine3

The extracellular matrix protein Reelin, secreted by Cajal-Retzius cells in the marginal zone of the cortex, controls the radial migration of cortical neurons. Reelin signaling involves the lipoprotein receptors apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR), the adapter protein Disabled1 (Dab1), and phosphatidylinositol-3-kinase (PI3K). Eventually, Reelin signaling acts on the cytoskeleton; however, these effects on cytoskeletal organization have remained elusive. In Reelin-deficient mutant mice, most cortical neurons are unable to migrate to their destinations, suggesting a role for Reelin signaling in the dynamic cytoskeletal reorganization that is required for neurons to migrate. Here, we show that Reelin signaling leads to serine3 phosphorylation of n-cofilin, an actin-depolymerizing protein that promotes the disassembly of F-actin. Phosphorylation at serine3 renders n-cofilin unable to depolymerize F-actin, thereby stabilizing the cytoskeleton. We provide evidence for ApoER2, Dab1, Src family kinases (SFKs), and PI3K to be involved in n-cofilin serine3 phosphorylation. Phosphorylation of n-cofilin takes place in the leading processes of migrating neurons as they approach the Reelin-containing marginal zone. Immunostaining for phospho-cofilin in dissociated reeler neurons is significantly increased after incubation in Reelin-containing medium compared with control medium. In a stripe choice assay, neuronal processes are stable on Reelin-coated stripes but grow on control stripes by forming lamellipodia. These novel findings suggest that Reelin-induced stabilization of neuronal processes anchors them to the marginal zone which appears to be required for the directional migration process.

Regulation of neural progenitor cell development in the nervous system

The mammalian telencephalon, which comprises the cerebral cortex, olfactory bulb, hippocampus, basal ganglia, and amygdala, is the most complex and intricate region of the CNS. It is the seat of all higher brain functions including the storage and retrieval of memories, the integration and processing of sensory and motor information, and the regulation of emotion and drive states. In higher mammals such as humans, the telencephalon also governs our creative impulses, ability to make rational decisions, and plan for the future. Despite its massive complexity, exciting work from a number of groups has begun to unravel the developmental mechanisms for the generation of the diverse neural cell types that form the circuitry of the mature telencephalon. Here, we review our current understanding of four aspects of neural development. We first begin by providing a general overview of the broad developmental mechanisms underlying the generation of neuronal and glial cell diversity in the telencephalon during embryonic development. We then focus on development of the cerebral cortex, the most complex and evolved region of the brain. We review the current state of understanding of progenitor cell diversity within the cortical ventricular zone and then describe how lateral signaling via the Notch-Delta pathway generates specific aspects of neural cell diversity in cortical progenitor pools. Finally, we review the signaling mechanisms required for development, and response to injury, of a specialized group of cortical stem cells, the radial glia, which act both as precursors and as migratory scaffolds for newly generated neurons.

Interaction between Reelin and Notch signaling regulates neuronal migration in the cerebral cortex

Neuronal migration is a fundamental component of brain development whose failure is associated with various neurological and psychiatric disorders. Reelin is essential for the stereotypical inside-out sequential lamination of the neocortex, but the molecular mechanisms of its action still remain unclear. Here we show that regulation of Notch activity plays an important part in Reelin-signal-dependent neuronal migration. We found that Reelin-deficient mice have reduced levels of the cleaved form of Notch intracellular domain (Notch ICD) and that loss of Notch signaling in migrating neurons results in migration and morphology defects. Further, overexpression of Notch ICD mitigates the laminar and morphological abnormalities of migrating neurons in Reeler. Finally, our in vitro biochemical studies show that Reelin signaling inhibits Notch ICD degradation via Dab1. Together, our results indicate that neuronal migration in the developing cerebral cortex requires a Reelin-Notch interaction.

Olfactory bulb interneurons releasing NO exhibit the Reelin receptor ApoEr2 and part of those targeted by NO express Reelin

Nitric oxide (NO) and Reelin both modulate neuronal plasticity in developing and mature synaptic networks. We recently showed a loss of neuronal nitric oxide synthase (nNOS) protein in the olfactory bulb of reeler mutants and advanced the hypothesis that the Reelin and NO signalling pathways may influence each other. We now studied the distribution of NO sensitive guanylyl cyclase (NOsGC), Reelin and its receptor Apolipoprotein E2 (ApoEr2) in the olfactory bulb by multiple fluorescence labelling and tested whether nNOS and ApoEr2 colocalize in this area. We also essayed the protein content of NOsGC in the reeler olfactory bulb and tested whether there are any changes in nNOS and NOsGC protein in other reeler brain areas. Olfactory bulb interneurons expressing ApoEr2 and nNOS are only few in the glomerular layer but represent the large majority of granule cell layer interneurons. Conversely, NOsGC interneurons are rare in the granule cell layer and abundant as periglomerular cells. Reelin containing periglomerular cells almost entirely belong to the NOsGC subset. These data further support the hypothesis of a reciprocal signalling between Reelin/NOsGC and ApoEr2/nNOS expressing neurons to affect olfactory bulb activity. We also show that a significant rise in NOsGC content accompanies the decrease of nNOS protein in the reeler olfactory bulb. The same reciprocal changes present in the cortex/striatum and the hippocampus of reeler mice. Thus, the influence that the deficit of extracellular Reelin seems to exert on nNOS and its receptor is not limited to the olfactory bulb but is a general feature of the reeler brain.

Reelin induces a radial glial phenotype in human neural progenitor cells by activation of Notch-1

Background

Reelin and Notch-1 signaling pathways have been recently found to be necessary to induce the expression of brain lipid binding protein (BLBP) and to promote the process extension and the maturation of the neuronal progenitors, the radial glial cells. In this study, we report the cross talk between these two pathways.

Results

Both in vitro Reelin treatment and overexpression of Notch-1 intracellular domain (NICD) induced BLBP expression and a radial glial phenotype in an immortalized human neural progenitor (HNP) cell line, isolated from the cortex of 14 weeks old fetus. Reelin treatment increased the level of NICD, indicating that Reelin signaling directly activates Notch-1. In addition, reducing NICD release, by inhibiting γ-secretase activity, inhibited the Reelin-induced radial glial phenotype in human neural progenitor cells. Furthermore, we found that Dab-1, an adaptor protein downstream of Reelin, was co-immunoprecipitated with Notch-1 and NICD.

Conclusion

These data indicate that Reelin signaling induces BLBP expression and a radial glial phenotype in human neural progenitor cells via the activation of Notch-1. This study suggest that Reelin signaling may act to fine tune Notch-1 activation to favor the induction of a radial glial phenotype prenataly and would thus offer an insight into how Notch-1 signaling leads to different cellular fates at different developmental stages.

Identification of erythroid-enriched gene expression in the mouse embryonic yolk sac using microdissected cells

Little is known about the genes that control the embryonic erythroid program. Laser capture microdissection was used to isolate primitive erythroid precursors and epithelial cells from frozen sections of the embryonic day 9.5 yolk sac. The RNA samples were amplified and labeled for hybridization to Affymetrix GeneChip Mouse Genome 430A 2.0 arrays. Ninety-one genes are expressed significantly higher in erythroid than in epithelial cells. Ingenuity pathway analysis indicates that many of these erythroid-enriched genes cluster in highly significant biological networks. One of these networks contains RBTN2/LMO2, SCL/TAL1, and EKLF/KLF1, three of the very few genes required for primitive erythropoiesis. Quantitative real-time polymerase chain reaction was used to verify that platelet factor 4, reelin, thrombospondin-1, and muscleblind-like 1 mRNA is erythroid-enriched. These genes have established roles in development or differentiation in other systems, and are, therefore, good candidates for regulating primitive erythropoiesis. These results provide a catalog of genes expressed during primitive erythropoiesis.

Reelin, a guidance signal for the regeneration of the entorhino-hippocampal path

The importance of reelin for cortical lamination in the developing CNS is well established, but its role in nerve fiber growth is not well understood. In this study, hippocampal slices were co-cultured with entorhinal slices of GFP mice, in order to compare the growth of the entorhino-hippocampal path in wild type and reeler mice. On day 6, regenerated fibers were seen to navigate from the entorhinal cortex into the hippocampus. The results showed that in wild type mice, regenerated fibers grew along the molecular layer of the dentate gyrus, and only a few fibers were found to penetrate through the granular layer into the hilus. This specific orientation was similar to the perforant path in vivo. Compared with perforant path regeneration in wild type mice, reeler mice seemed to have lost their specific orientation and proper termination in the hippocampus. Without the guidance signal from reelin, the regenerated fibers left the molecular layers and continued to grow aberrantly, i.e., in the granular layer, hilus, pyramidal layer and even stratum oriens. Particularly in the dentate gyrus, the fibers meandered around the cells in the hilus and resembled a network. The study concludes that reelin also serves as an important guidance signal for neuroregeneration of the perforant path.

Expression of reelin, its receptors and its intracellular signaling protein, Disabled1 in the canary brain: relationships with the song control system

Songbirds produce learned vocalizations that are controlled by a specialized network of neural structures, the song control system. Several nuclei in this song control system demonstrate a marked degree of adult seasonal plasticity. Nucleus volume varies seasonally based on changes in cell size or spacing, and in the case of nucleus HVC and area X on the incorporation of new neurons. Reelin, a large glycoprotein defective in reeler mice, is assumed to determine the final location of migrating neurons in the developing brain. In mammals, reelin is also expressed in the adult brain but its functions are less well characterized. We investigated the relationships between the expression of reelin and/or its receptors and the dramatic seasonal plasticity in the canary (Serinus canaria) brain. We detected a broad distribution of the reelin protein, its mRNA and the mRNAs encoding for the reelin receptors (VLDLR and ApoER2) as well as for its intracellular signaling protein, Disabled1. These different mRNAs and proteins did not display the same neuroanatomical distribution and were not clearly associated, in an exclusive manner, with telencephalic brain areas that incorporate new neurons in adulthood. Song control nuclei were associated with a particular specialized expression of reelin and its mRNA, with the reelin signal being either denser or lighter in the song nucleus than in the surrounding tissue. The density of reelin-immunoreactive structures did not seem to be affected by 4 weeks of treatment with exogenous testosterone. These observations do not provide conclusive evidence that reelin plays a prominent role in the positioning of new neurons in the adult canary brain but call for additional work on this protein analyzing its expression comparatively during development and in adulthood with a better temporal resolution at critical points in the reproductive cycle when brain plasticity is known to occur.

Genes, plasticity and mental retardation

Functional and structural plasticity is a fundamental property of the brain involved in diverse processes ranging from brain construction and repair to storage of experiences during lifetime. Our current understanding of different forms of brain plasticity mechanisms has advanced tremendously in the last decades, benefiting from studies of development and memory storage in adulthood and from investigations of diverse diseased conditions. In this review, we focus on the role of mental retardation (MR) genes and show how this developing area of research can enrich our knowledge of the cellular and molecular mechanisms of brain plasticity and cognitive functions, and of the dysfunctional mechanisms underlying MR. We describe two main groups of MR genes; those leading to dysfunctional neurodevelopmental programs and brain malformations, and those which rely on alterations in molecular mechanisms underlying synaptic organization and plasticity. We first explore the role of MR genes in key mechanisms of neurogenesis and neuronal migration during development and in the adult, such as actin and microtubule-cytoskeletal dynamics and signal transduction. We then define the contribution of MR genes to forms of activity-dependent synaptic modifications, such as those involved in molecular organization of the synapse, intracellular signaling regulating gene programs and neuronal cytoskeleton to control network remodeling. We trace the characteristics of MR genes playing key roles in many forms of brain plasticity mechanisms, and highlight specific MR genes that endorse distinct roles in different cell types or brain regions, and at various times of a brain lifetime.

Epigenetic mechanisms mediating the long-term effects of maternal care on development

The long-term consequences of early environmental experiences for development have been explored extensively in animal models to better understand the mechanisms mediating risk of psychopathology in individuals exposed to childhood adversity. One common feature of these models is disruption of the mother-infant relationship which is associated with impairments in stress responsivity and maternal behavior in adult offspring. These behavioral and physiological characteristics are associated with stable changes in gene expression which emerge in infancy and are sustained into adulthood. Recent evidence suggests that these long-term effects may be mediated by epigenetic modification to the promoter regions of steroid receptor genes. In particular, DNA methylation may be critical to maternal effects on gene expression and thus generate phenotypic differentiation of offspring and, through effects on maternal behavior of offspring, mediate the transmission of these effects across generations. In this review we explore evidence for the influence of mother-infant interactions on the epigenome and consider evidence for and the implications of such epigenetic effects for human mental health.

Histogenetic processes leading to the laminated neocortex: migration is only a part of the story

The principal events of neocortical histogenesis were anticipated by work published prior to the 20th century. These were neuronal proliferation and migration and the complex events of cortical pattern formation leading to a laminated architecture where each layer is dominated by a principal neuronal class. Work that has followed has extended the knowledge of the workings of the proliferative epithelium, cellular mechanisms of migration and events through which cells are winnowed and then differentiate once their postmigratory positions are established. Work yet ahead will emphasize mechanisms that coordinate the molecular events that integrate proliferation and cell class specification in relation to the final neocortical neural system map.

The neurobiology of autism

Improving clinical tests are allowing us to more precisely classify autism spectrum disorders and diagnose them at earlier ages. This raises the possibility of earlier and potentially more effective therapeutic interventions. To fully capitalize on this opportunity, however, will require better understanding of the neurobiological changes underlying this devastating group of developmental disorders. It is becoming clear that the normal trajectory of neurodevelopment is altered in autism, with aberrations in brain growth, neuronal patterning and cortical connectivity. Changes to the structure and function of synapses and dendrites have also been strongly implicated in the pathology of autism by morphological, genetic and animal modeling studies. Finally, environmental factors are likely to interact with the underlying genetic profile, and foster the clinical heterogeneity seen in autism spectrum disorders. In this review we attempt to link the molecular pathways altered in autism to the neurodevelopmental and clinical changes that characterize the disease. We focus on signaling molecules such as neurotrophin, Reelin, PTEN and hepatocyte growth factor, neurotransmitters such as serotonin and glutamate, and synaptic proteins such as neurexin, SHANK and neuroligin. We also discuss evidence implicating oxidative stress, neuroglial activation and neuroimmunity in autism.

Divergent roles of ApoER2 and Vldlr in the migration of cortical neurons

Reelin, its lipoprotein receptors [very low density lipoprotein receptor (Vldlr) and apolipoprotein E receptor 2 (ApoER2; also known as Lrp8)], and the cytoplasmic adaptor protein disabled 1 (Dab1) are important for the correct formation of layers in the cerebral cortex. Reeler mice lacking the reelin protein show altered radial neuronal migration resulting in an inversion of cortical layers. ApoER2 Vldlr double-knockout mutants and Dab1 mutants show a reeler-like phenotype, whereas milder phenotypes are found if only one of the two lipoprotein receptors for reelin is absent. However, the precise role of the individual reelin receptors in neuronal migration remained unclear. In the study reported here, we performed fate mapping of newly generated cortical neurons in single and double receptor mutants using bromodeoxyuridine-labeling and layer-specific markers. We present evidence for divergent roles of the two reelin receptors Vldlr and ApoER2, with Vldlr mediating a stop signal for migrating neurons and ApoER2 being essential for the migration of late generated neocortical neurons.

Lissencéphalies : aspects cliniques et génétiques

The term lissencephaly covers a group of rare malformations sharing the common feature of anomalies in the appearance of brain convolutions (characterised by simplification or absence of folding) associated with abnormal organisation of the cortical layers as a result of neuronal migration defects during embryogenesis. Children with lissencephaly have feeding and swallowing problems, muscle tone anomalies (early hypotonia and subsequently limb hypertonia), seizures (in particular, infantile spasms) and severe psychomotor retardation. Multiple forms of lissencephaly have been described and their current classification is based on the associated malformations and underlying aetiology. Two large groups can be distinguished: classical lissencephaly (and its variants) and cobblestone lissencephaly. In classical lissencephaly (or type I), the cortex appears thickened, with four more or less disorganised layers rather than six normal layers. In the variants of classical lissencephaly, extra-cortical anomalies are also present (total or subtotal agenesis of the corpus callosum and/or cerebellar hypoplasia). The classical lissencephalies and the variant forms can be further divided into several subgroups. Four forms can be distinguished on the basis of their genetic aetiology: anomalies in the LIS1 gene (isolated lissencephaly and Miller-Dieker syndrome), anomalies in the TUBA3 and DCX genes, and lissencephalies caused by mutations in the ARX gene (XLAG syndrome, X-linked lissencephaly with agenesis of the corpus callosum). The incidence of all forms of type I lissencephaly is around 1 in 100,000 births. In addition to these four entities, isolated lissencephalies without a known genetic defect, lissencephalies with severe microcephaly (microlissencephaly) and lissencephalies associated with polymalformative syndromes are also included in the group of classical lissencephalies. Cobblestone lissencephaly (formally referred to as type II) is present in three entities: the Walker-Warburg, Fukuyama and MEB (Muscle-Eye-Brain) syndromes. It is characterised by global disorganisation of cerebral organogenesis with an uneven cortical surface (with a pebbled or cobblestone appearance). Microscopic examination reveals total disorganisation of the cortex and the absence of any distinguishable layers. Management is symptomatic only (swallowing problems require adapted feeding to prevent food aspiration, articular and respiratory physiotherapy to prevent orthopaedic problems resulting from hyptonia and treatment of gastrooesophageal reflux). The epilepsy is often resistant to treatment. The encephalopathy associated with lissencephaly is often very severe and affected children are completely dependent on the carer.

Reelin depletion in the entorhinal cortex of human amyloid precursor protein transgenic mice and humans with Alzheimer's disease

Reelin regulates nervous system development and modulates synaptic plasticity in the adult brain. Several findings suggest that alterations in Reelin signaling may contribute to neuronal dysfunction associated with Alzheimer's disease (AD). Cell surface receptors for Reelin, including integrins and very-low-density lipoprotein receptor/apolipoprotein E2 receptor, may be targets of amyloid-beta (Abeta) peptides presumed to play key roles in the pathogenesis of AD. Reelin also regulates the extent of tau phosphorylation. Finally, increased amounts of Reelin fragments have been found in CSF from AD patients, suggesting altered processing of Reelin. We therefore hypothesized that Reelin levels might be altered in the brains of human amyloid precursor protein (hAPP) transgenic mice, particularly in brain regions vulnerable to AD such as hippocampus and entorhinal cortex. Compared with nontransgenic controls, hAPP mice had significantly fewer Reelin-expressing pyramidal cells in the entorhinal cortex, the major population of glutamatergic neurons expressing Reelin in the brain. Western blot analysis of the hippocampus, which receives projections from the entorhinal cortex, revealed significant reductions in Reelin levels. In contrast, the number of Reelin-expressing GABAergic interneurons was not altered in either the entorhinal cortex or the hippocampus. Thus, neuronal expression of hAPP/Abeta is sufficient to reduce Reelin expression in a specific population of entorhinal cortical pyramidal neurons in vivo. Underscoring the relevance of these findings, we found qualitatively similar reductions of Reelin-expressing pyramidal neurons in the entorhinal cortex of AD brains. We conclude that alterations in Reelin processing or signaling may be involved in AD-related neuronal dysfunction.

The superficial layers of the superior colliculus are cytoarchitectually and myeloarchitectually disorganized in the reelin-deficient mouse, reeler

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.

Nitric oxide synthase protein levels, not the mRNA, are downregulated in olfactory bulb interneurons of reeler mice

Homozygous mutations in the Reelin gene result in severe disruption of brain development. The histogenesis of layered regions, like the neocortex, hippocampus and the cerebellum, is most notably affected in mouse reeler mutants and similar traits are also present in mice lacking molecular components of the Reelin signalling pathway. Moreover, there is evidence for an additional role of Reelin in sustaining synaptic plasticity in adult networks. Nitric oxide is an important gaseous messenger that can modulate neuronal plasticity both in developing and mature synaptic networks and has been shown to facilitate synaptic changes in the hippocampus, cerebellum and olfactory bulb. We studied the distribution and content of neuronal nitric oxide synthase in the olfactory bulbs of reeler and wildtype mice. Immunocytochemistry reveals that Reelin and neuronal nitric oxide synthase containing interneurons are two distinct, non overlapping cell populations of the olfactory bulb. We show by in situ hybridization that both nitrergic and Reelin expressing cells represent only a subset of olfactory bulb GABAergic neurons. Immunoblots show that neuronal nitric oxide synthase protein content is decreased by two thirds in reeler mice causing a detectable loss of immunolabelled cells throughout the olfactory bulb of this strain. However, neuronal nitric oxide synthase mRNA levels, essayed by quantitative real-time RT-PCR, are unaffected in the reeler olfactory bulb. Thus, disruption of the Reelin signalling pathway may modify the turnover of neuronal nitric oxide synthase in the olfactory bulb and possibly affects nitric oxide functions in reeler mice.

Early pharmacological treatment of autism: a rationale for developmental treatment

Autism is a dynamic neurodevelopmental syndrome in which disabilities emerge during the first three postnatal years and continue to evolve with ongoing development. We briefly review research in autism describing subtle changes in molecules important in brain development and neurotransmission, in morphology of specific neurons, brain connections, and in brain size. We then provide a general schema of how these processes may interact with particular emphasis on neurotransmission. In this context, we present a rationale for utilizing pharmacologic treatments aimed at modifying key neurodevelopmental processes in young children with autism. Early treatment with selective serotonin reuptake inhibitors (SSRIs) is presented as a model for pharmacologic interventions because there is evidence in autistic children for reduced brain serotonin synthesis during periods of peak synaptogenesis; serotonin is known to enhance synapse refinement; and exploratory studies with these agents in autistic children exist. Additional hypothetical developmental interventions and relevant published clinical data are described. Finally, we discuss the importance of exploring early pharmacologic interventions within multiple experimental settings in order to develop effective treatments as quickly as possible while minimizing risks.

Prevention of stroke and dementia by statin therapy: Experimental and clinical evidence of their pleiotropic effects

Stroke and dementia are major causes of disability in most countries. Epidemiological studies have demonstrated that statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) are likely to reduce the risk for developing these formidable disorders. The favorable outcomes in statin users may be attributable to not only cholesterol-dependent actions, but also various cholesterol-independent actions called "pleiotropic effects." Several clinical trials have suggested that statins decrease the incidence of stroke, especially ischemic stroke. Statins improve endothelial function, inhibit platelet activation, reduce blood coagulability, and suppress inflammatory reactions, all of which may contribute to the beneficial effects of the therapy. Statins also reduce the risk of vasospasm caused by subarachnoid hemorrhage (SAH). In addition, statins might inhibit the development and progression of Alzheimer's disease (AD), the dominant type of dementia in most industrialized countries, upstream of the amyloid cascade. In vitro studies have shown that statins modulate the metabolism of the beta-amyloid precursor protein (APP) and reduce the extracellular level of its proteolytic product, amyloid-beta (Abeta). The aggregated Abeta is cytotoxic, leading to formation of neurofibrillary tangles and neuronal loss in the brain. Inflammatory processes are active in AD and may contribute significantly to AD pathology. We review the experimental background regarding the pleiotropic effects of statins and summarize clinical trials that examined the preventative effects of statin therapy on stroke and dementia. We include current trials in which statin therapy is initiated within 24 hr of onset of acute ischemic stroke.

The dentate mossy fibers: structural organization, development and plasticity

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

Development of cell and fiber layers in the dentate gyrus

This chapter deals with the laminated organization of the dentate gyrus, particularly with the molecular signals controlling its development. First, sites of granule cell generation, their modes and routes of migration are described. This is followed by an analysis of the molecular determinants governing the formation of a tightly packed granule cell layer that is normal in rodents and primates. Reelin, a protein of the extracellular matrix, plays an important role for the proper migration and lamination of the granule cells during development and for the maintenance of a laminated dentate gyrus in adulthood. Granule cell positioning is crucial for the laminated termination of commissural/associational fibers to the dentate gyrus, suggesting that the granule cells carry positional signals for these fibers. In contrast, not signals of the target cells but molecules of the extracellular matrix, such as hyaluronan, underlie the layer-specific termination of fibers from the entorhinal cortex. The molecular determinants controlling axonal pathfinding and target recognition of the profusely terminating cholinergic and GABAergic subcortical afferents still need to be elucidated.

Retrograde infection of precerebellar nuclei neurons by injection of a recombinant adenovirus into the cerebellar cortex of normal and reeler mice

The reeler mouse is an autosomal recessive mutant mouse caused by mutation of the reelin gene and characterized by cerebellar ataxia. To determine whether the distribution pattern of precerebellar nuclei neurons in the brainstem of the reeler mouse changes, we injected a small volume of a replication-defective recombinant adenovirus carrying E. coli beta-galactosidase (lacZ) into the cerebellar cortex of normal and reeler mice. Five days later, the mice were transcardially perfused by a fixative solution. X-gal staining of coronal or sagittal sections of the brainstem revealed that many origins for reticulocerebellar, cuneocerebellar, trigeminocerebellar, and pontocerebellar projections were retrogradely labeled, but only a few olivocerebellar neurons were labeled. Retrogradely labeled neurons in the lateral reticular nucleus tended to locate more laterally and be more condensed into a small compartment in the reeler compared with their normal counterparts. Retrogradely labeled neurons in the external cuneate nucleus were more dorsally shifted in the reeler mice compared with their normal counterparts. We could not find any differences between the normal and reeler mice in the distribution patterns of their trigeminocerebellar projection neurons. Retrogradely labeled pontocerebellar neurons in the basilar pons of the reeler mouse were reduced in number compared with their normal counterparts in addition to being more ventrally and laterally shifted. These findings strongly suggest that the migration of some precerebellar nuclei neurons from the rhombic lip to their final loci may be obstructed in the reeler mice.

Neuroanatomical phenotypes in the reeler mouse

The reeler mouse (Reln) has been proposed as a neurodevelopmental model for certain neurological and psychiatric conditions and has been studied by qualitative histochemistry and electron microscopy. Using magnetic resonance microscopy (MRM), we have quantitated for the first time the neuromorphology of Reln mice at a resolution of 21.5 microm. The neuroanatomical phenotypes of heterozygous and homozygous mutant Reln mice were compared to those of wild type (WT) littermates using morphometry and texture analysis. The cortical, hippocampal, and cerebellar phenotypes of the heterozygous and homozygous mutant Reln mice were confirmed, and new features were revealed. The Reln(rl/rl) mice possessed a smaller brain, and both Reln(rl/+) and Reln(rl/rl) mice had increased ventricles compared to WT controls. Shape differences were found between WT and Reln(rl/rl) brains, specifically in cerebellum, olfactory bulbs, dorsomedial frontal and parietal cortex, certain regions of temporal and occipital lobes, as well as in the lateral ventricles and ventral hippocampus. These findings suggest that certain brain regions may be more severely impacted by the Reln mutation than others. Gadolinium-based active staining demonstrated that layers of the hippocampus were disorganized in Reln(rl/rl) mice and differences in thickness of these layers were identified between WT and Reln(rl/rl) mice. The intensity distributions characteristic to the dorsal, middle, and ventral hippocampus were altered in the Reln(rl/rl), especially in the ventral hippocampus. These differences were quantified using skewness and modeling the intensity distributions with a Gaussian mixture. Our results suggest that structural features of Reln(rl/rl) brain most closely phenocopy those of patients with Norman-Roberts lissencephaly.