A role of MAP1B in Reelin-dependent neuronal migration

GSK3 Function in the Brain during Development, Neuronal Plasticity, and Neurodegeneration

GSK3 has diverse functions, including an important role in brain pathology. In this paper, we address the primary functions of GSK3 in development and neuroplasticity, which appear to be interrelated and to mediate age-associated neurological diseases. Specifically, GSK3 plays a pivotal role in controlling neuronal progenitor proliferation and establishment of neuronal polarity during development, and the upstream and downstream signals modulating neuronal GSK3 function affect cytoskeletal reorganization and neuroplasticity throughout the lifespan. Modulation of GSK3 in brain areas subserving cognitive function has become a major focus for treating neuropsychiatric and neurodegenerative diseases. As a crucial node that mediates a variety of neuronal processes, GSK3 is proposed to be a therapeutic target for restoration of synaptic functioning and cognition, particularly in Alzheimer's disease.

Deconstructing GSK-3: The Fine Regulation of Its Activity

Glycogen synthase kinase-3 (GSK-3) unique position in modulating the function of a diverse series of proteins in combination with its association with a wide variety of human disorders has attracted significant attention to the protein both as a therapeutic target and as a means to understand the molecular basis of these disorders. GSK-3 is ubiquitously expressed and, unusually, constitutively active in resting, unstimulated cells. In mammals, GSK-3α and β are each expressed widely at both the RNA and protein levels although some tissues show preferential levels of some of the two proteins. Neither gene appears to be acutely regulated at the transcriptional level, whereas the proteins are controlled posttranslationally, largely through protein-protein interactions or by posttranslational regulation. Control of GSK-3 activity thus occurs by complex mechanisms that are each dependent upon specific signalling pathways. Furthermore, GSK-3 appears to be a cellular nexus, integrating several signalling systems, including several second messengers and a wide selection of cellular stimulants. This paper will focus on the different ways to control GSK-3 activity (phosphorylation, protein complex formation, truncation, subcellular localization, etc.), the main signalling pathways involved in its control, and its pathological deregulation.

Go or stop? Divergent roles of Reelin in radial neuronal migration

Neuronal migration is an essential step of brain development and is controlled by a variety of cellular proteins and extracellular matrix molecules. Reelin, an extracellular matrix protein, is required for neuronal migration. Over the past 10 years, the Reelin signaling cascade has been studied intensively. However, the role of Reelin in neuronal migration has remained unclear. Different Reelin fragments and different Reelin receptors suggest multiple functions of Reelin. In this review, the authors focus on Reelin effects on the actin cytoskeleton of migrating neurons.

Guiding neuronal cell migrations

Neuronal migration is, along with axon guidance, one of the fundamental mechanisms underlying the wiring of the brain. As other organs, the nervous system has acquired the ability to grow both in size and complexity by using migration as a strategy to position cell types from different origins into specific coordinates, allowing for the generation of brain circuitries. Guidance of migrating neurons shares many features with axon guidance, from the use of substrates to the specific cues regulating chemotaxis. There are, however, important differences in the cell biology of these two processes. The most evident case is nucleokinesis, which is an essential component of migration that needs to be integrated within the guidance of the cell. Perhaps more surprisingly, the cellular mechanisms underlying the response of the leading process of migrating cells to guidance cues might be different to those involved in growth cone steering, at least for some neuronal populations.

MAP1B regulates axonal development by modulating Rho-GTPase Rac1 activity

This article shows a novel function for the MAP1B protein, related to the control of actin dynamics through interaction with Tiam1.

Cultured neurons obtained from MAP1B-deficient mice have a delay in axon outgrowth and a reduced rate of axonal elongation compared with neurons from wild-type mice. Here we show that MAP1B deficiency results in a significant decrease in Rac1 and cdc42 activity and a significant increase in Rho activity. We found that MAP1B interacted with Tiam1, a guanosine nucleotide exchange factor for Rac1. The decrease in Rac1/cdc42 activity was paralleled by decreases in the phosphorylation of the downstream effectors of these proteins, such as LIMK-1 and cofilin. The expression of a constitutively active form of Rac1, cdc42, or Tiam1 rescued the axon growth defect of MAP1B-deficient neurons. Taken together, these observations define a new and crucial function of MAP1B that we show to be required for efficient cross-talk between microtubules and the actin cytoskeleton during neuronal polarization.

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.

Reelin regulates postnatal neurogenesis and enhances spine hypertrophy and long-term potentiation

Reelin, an extracellular protein essential for neural migration and lamination, is also expressed in the adult brain. To unravel the function of this protein in the adult forebrain, we generated transgenic mice that overexpress Reelin under the control of the CaMKIIalpha promoter. Overexpression of Reelin increased adult neurogenesis and impaired the migration and positioning of adult-generated neurons. In the hippocampus, the overexpression of Reelin resulted in an increase in synaptic contacts and hypertrophy of dendritic spines. Induction of long-term potentiation (LTP) in alert-behaving mice showed that Reelin overexpression evokes a dramatic increase in LTP responses. Hippocampal field EPSP during a classical conditioning paradigm was also increased in these mice. Our results indicate that Reelin levels in the adult brain regulate neurogenesis and migration, as well as the structural and functional properties of synapses. These observations suggest that Reelin controls developmental processes that remain active in the adult brain.

Evolution of the spatial distribution of MAP1B phosphorylation sites in vertebrate neurons

The microtubule-associated protein MAP1B has important roles in neural development, particularly in migrating and differentiating neurons. MAP1B is phosphorylated by glycogen synthase kinase 3beta (GSK-3beta) at a site that requires prior phosphorylation by another kinase four amino acid residues downstream of the GSK-3beta site, a so-called primed site, and at non-primed sites that have no such requirement. In developing mammalian neurons, MAP1B phosphorylated by GSK-3beta at primed and non-primed sites is distributed in spatially distinct patterns. Non-primed GSK-3beta-phosphorylated MAP1B sites are only expressed in axons and are present in the form of a gradient that is highest distally, towards the growth cone. In contrast, primed GSK-3beta-phosphorylated MAP1B sites are present throughout the neuron including the somato-dendritic compartment and uniformly throughout the axon. To examine the function of these two sites, we explored the evolutionary conservation of the spatial distribution of GSK-3beta primed and non-primed sites on MAP1B in vertebrate neurons. We immunostained spinal cord sections from embryonic or newly hatched representatives of all of the main vertebrate groups using phospho-specific antibodies to GSK-3beta primed and non-primed sites on MAP1B. This revealed a remarkable evolutionary conservation of the distribution of primed and non-primed GSK-3beta-phosphorylated MAP1B sites in developing vertebrate neurons. By analysing amino acid sequences of MAP1B we found that non-primed GSK-3beta sites are more highly conserved than primed sites throughout the vertebrates, suggesting that the latter evolved later. Finally, distinct distribution patterns of GSK-3beta primed and non-primed sites on MAP1B were preserved in cultured rat embryonic cortical neurons, opening up the possibility of studying the two sites in vitro.

Neurites regrowth of cortical neurons by GSK3beta inhibition independently of Nogo receptor 1

Lesioned axons do not regenerate in the adult mammalian CNS, owing to the over-expression of inhibitory molecules such as myelin-derived proteins or chondroitin sulphate proteoglycans. In order to overcome axon inhibition, strategies based on extrinsic and intrinsic treatments have been developed. For myelin-associated inhibition, blockage with NEP1-40, receptor bodies or IN-1 antibodies has been used. In addition, endogenous blockage of cell signalling mechanisms induced by myelin-associated proteins is a potential tool for overcoming axon inhibitory signals. We examined the participation of glycogen synthase kinase 3beta (GSK3beta) and extracellular-related kinase (ERK) 1/2 in axon regeneration failure in lesioned cortical neurons. We also investigated whether pharmacological blockage of GSK3beta and ERK1/2 activities facilitates regeneration after myelin-directed inhibition in two models: (i) cerebellar granule cells and (ii) lesioned entorhino-hippocampal pathway in slice cultures, and whether the regenerative effects are mediated by Nogo Receptor 1 (NgR1). We demonstrate that, in contrast to ERK1/2 inhibition, the pharmacological treatment of GSK3beta inhibition strongly facilitated regrowth of cerebellar granule neurons over myelin independently of NgR1. Finally, these regenerative effects were corroborated in the lesioned entorhino-hippocampal pathway in NgR1-/- mutant mice. These results provide new findings for the development of new assays and strategies to enhance axon regeneration in injured cortical connections.

Nervous-tissue-specific elimination of microtubule-actin crosslinking factor 1a results in multiple developmental defects in the mouse brain

The microtubule-actin crosslinking factor 1 (MACF1) is a ubiquitous cytoskeletal linker protein with multiple spliced isoforms expressed in different tissues. The MACF1a isoform contains microtubule and actin-binding regions and is expressed at high levels in the nervous system. Macf1-/- mice are early embryonic lethal and hence the role of MACF1 in the nervous system could not be determined. We have specifically knocked out MACF1a in the developing mouse nervous system using Cre/loxP technology. Mutant mice died within 24-36h after birth of apparent respiratory distress. Their brains displayed a disorganized cerebral cortex with a mixed layer structure, heterotopia in the pyramidal layer of the hippocampus, disorganized thalamocortical and corticofugal fibers, and aplastic anterior and hippocampal commissures. Embryonic neurons showed a defect in traversing the cortical plate. Our data suggest a critical role for MACF1 in neuronal migration that is dependent on its ability to interact with both microfilaments and microtubules.

Reelin promotes neuronal orientation and dendritogenesis during preplate splitting

The secreted ligand Reelin is thought to regulate the translocation and positioning of prospective layer 6 (L6) neurons into the preplate, a plexus of neurons overlying the ventricular zone. We examined wild type and Reelin-deficient cortices and found that L6 neurons were equivalently positioned beneath the pia during the period of preplate splitting and initial cortical plate (CP) formation. The absence of detectable L6 ectopia in "reeler" cortices at this developmental time point indicates that Reelin-signaling might not regulate L6 neuron migration or gross positioning during preplate splitting. To explore the acute response of L6 neurons to Reelin, subpial injections of Reelin were made into Reelin-deficient explants. Reelin injection caused L6 neurons to orient their nuclei and polarize their Golgi toward the pia while initiating exuberant dendritic (MAP2+) outgrowth within 4 h. This rapid Reelin-dependent neuronal orientation and alignment created CP-like histology without any significant change in the mean position of the population of L6 neurons. Conversely, subplate cells and chondroitin sulfate proteoglycan immunoreactivity were found at significantly deeper positions from the pial surface after injection, suggesting that Reelin partially rescues preplate splitting within 4 h. Thus, Reelin has a direct role in promoting rapid morphological differentation and orientation of L6 neurons during preplate splitting.

Differential gene expression analysis of human entorhinal cortex support a possible role of some extracellular matrix proteins in the onset of Alzheimer disease

The onset of Alzheimer's disease (AD) has been associated with the specific vulnerability of neurons in the upper layers of the entorhinal cortex. To define the molecular characteristics of those neurons, we have used microarrays to define the gene expression in that region. In this way, we identified several genes that are expressed distinctly in the upper and lower layers of the entorhinal cortex. These include the genes encoding the matrix Gla protein, collagen type 1alpha2, reelin, semaphorin 3C or the relaxin receptor, all related to the extracellular matrix. Thus, differences in the extracellular matrix components between the upper and lower layers of the entorhinal cortex may in part explain the vulnerability of neurons present in the upper layers of this brain region in disorders like AD.

Kidins220/ARMS modulates the activity of microtubule-regulating proteins and controls neuronal polarity and development

In order for neurons to perform their function, they must establish a highly polarized morphology characterized, in most of the cases, by a single axon and multiple dendrites. Herein we find that the evolutionarily conserved protein Kidins220 (kinase D-interacting substrate of 220-kDa), also known as ARMS (ankyrin repeat-rich membrane spanning), a downstream effector of protein kinase D and neurotrophin and ephrin receptors, regulates the establishment of neuronal polarity and development of dendrites. Kidins220/ARMS gain and loss of function experiments render severe phenotypic changes in the processes extended by hippocampal neurons in culture. Although Kidins220/ARMS early overexpression hinders neuronal development, its down-regulation by RNA interference results in the appearance of multiple longer axon-like extensions as well as aberrant dendritic arbors. We also find that Kidins220/ARMS interacts with tubulin and microtubule-regulating molecules whose role in neuronal morphogenesis is well established (microtubule-associated proteins 1b, 1a, and 2 and two members of the stathmin family). Importantly, neurons where Kidins220/ARMS has been knocked down register changes in the phosphorylation activity of MAP1b and stathmins. Altogether, our results indicate that Kidins220/ARMS is a key modulator of the activity of microtubule-regulating proteins known to actively regulate neuronal morphogenesis and suggest a mechanism by which it contributes to control neuronal development.

Nonprimed and DYRK1A-primed GSK3 beta-phosphorylation sites on MAP1B regulate microtubule dynamics in growing axons

MAP1B is a developmentally regulated microtubule-associated phosphoprotein that regulates microtubule dynamics in growing axons and growth cones. We used mass spectrometry to map 28 phosphorylation sites on MAP1B, and selected for further study a putative primed GSK3 beta site and compared it with two nonprimed GSK3 beta sites that we had previously characterised. We raised a panel of phosphospecific antibodies to these sites on MAP1B and used it to assess the distribution of phosphorylated MAP1B in the developing nervous system. This showed that the nonprimed sites are restricted to growing axons, whereas the primed sites are also expressed in the neuronal cell body. To identify kinases phosphorylating MAP1B, we added kinase inhibitors to cultured embryonic cortical neurons and monitored MAP1B phosphorylation with our panel of phosphospecific antibodies. These experiments identified dual-specificity tyrosine-phosphorylation-regulated kinase (DYRK1A) as the kinase that primes sites of GSK3 beta phosphorylation in MAP1B, and we confirmed this by knocking down DYRK1A in cultured embryonic cortical neurons by using shRNA. DYRK1A knockdown compromised neuritogenesis and was associated with alterations in microtubule stability. These experiments demonstrate that MAP1B has DYRK1A-primed and nonprimed GSK3 beta sites that are involved in the regulation of microtubule stability in growing axons.

GSK3beta in ethanol neurotoxicity

Alcohol consumption during pregnancy is a significant public health problem and may result in a wide range of adverse outcomes for the child. The developing central nervous system (CNS) is particularly susceptible to ethanol toxicity. Children with fetal alcohol spectrum disorders (FASD) have a variety of cognitive, behavioral, and neurological impairments. FASD currently represents the leading cause of mental retardation in North America ahead of Down syndrome and cerebral palsy. Ethanol exposure during development causes multiple abnormalities in the brain such as permanent loss of neurons, ectopic neurons, and alterations in synaptogenesis and myelinogenesis. These alcohol-induced structural alterations in the developing brain underlie many of the behavioral deficits observed in FASD. The cellular and molecular mechanisms of ethanol neurotoxicity, however, remain unclear. Ethanol elicits cellular stresses, including oxidative stress and endoplasmic reticulum stress. Glycogen synthase kinase 3beta (GSK3beta), a multifunctional serine/threonine kinase, responds to various cellular stresses. GSK3beta is particularly abundant in the developing CNS, and regulates diverse developmental events in the immature brain, such as neurogenesis and neuronal differentiation, migration, and survival. Available evidence indicates that the activity of GSK3beta in the CNS is affected by ethanol. GSK3beta inhibition provides protection against ethanol neurotoxicity, whereas high GSK3beta activity/expression sensitizes neuronal cells to ethanol-induced damages. It appears that GSK3beta is a converging signaling point that mediates some of ethanol's neurotoxic effects.

Fast-twitch sarcomeric and glycolytic enzyme protein loss in inclusion body myositis

Inclusion body myositis (IBM) is an inflammatory disease of skeletal muscle of unknown cause. To further understand the nature of the tissue injury in this disease, we developed methods for large-scale detection and quantitation of proteins in muscle biopsy samples and analyzed proteomic data produced by these methods together with histochemical, immunohistochemical, and microarray data. Twenty muscle biopsy samples from patients with inflammatory myopathies (n = 17) or elderly subjects without neuromuscular disease (n = 3) were profiled by proteomic studies using liquid chromatographic separation of peptides followed by mass spectrometry. Thirteen of the diseased samples additionally underwent microarray studies. Seventy muscle specimens from patients with a range of neuromuscular disorders were examined by ATPase histochemical methods. Smaller numbers of samples underwent immunohistochemical and immunoblot studies. Mass spectrometric studies identified and quantified approximately 300 total distinct proteins in each muscle sample. In IBM and to a lesser extent in polymyositis, proteomic studies confirmed by histochemical, immunohistochemical, and immunoblot studies showed loss of many fast-twitch specific structural proteins and glycolytic enzymes despite relative preservation of transcript levels. Increased abundance of a nuclear membrane protein, immunoglobulins, and two calpain-3 substrates were present. The atrophy present in IBM muscle is accompanied by preferential loss of fast-twitch structural proteins and glycolytic enzymes, particularly glycogen debranching enzyme, with relative preservation of the abundance of their respective transcripts. Although muscle atrophy has long been recognized in IBM, these studies are the first to report specific proteins which are reduced in quantity in IBM muscle.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.

Association analysis of schizophrenia on 18 genes involved in neuronal migration: MDGA1 as a new susceptibility gene

Several lines of evidence support the theory of schizophrenia (SZ) being a neurodevelopmental disorder. The structural, cytoarchitectural and functional brain abnormalities reported in patients with SZ, might be due to aberrant neuronal migration, since the final position of neurons affects neuronal function, morphology, and formation of synaptic connections. We have investigated the putative association between SZ and gene variants engaged in the neuronal migration process, by performing an association study on 839 cases and 1,473 controls of Scandinavian origin. Using a gene-wide approach, tagSNPs in 18 candidate genes have been genotyped, with gene products involved in the neuron-to-glial cell adhesion, interactions with the DISC1 protein and/or rearrangements of the cytoskeleton. Of the 289 markers tested, 19 markers located in genes MDGA1, RELN, ITGA3, DLX1, SPARCL1, and ASTN1, attained nominal significant P-values (P < 0.05) in either a genotypic or allelic association test. All of these genes, except transcription factor DLX1, are involved in the adhesion between neurons and radial glial cells. Eight markers obtained nominal significance in both tests, and were located in intronic or 3'UTR regions of adhesion molecule MDGA1 and previously reported SZ candidate RELN. The most significant result was attained for MDGA1 SNP rs9462341 (unadjusted association results: genotypic P = 0.00095; allelic P = 0.010). Several haplotypes within MDGA1, RELN, ITGA3, and ENAH were nominally significant. Further studies in independent samples are needed, including upcoming genome wide association study results, but our data suggest that MDGA1 is a new SZ susceptibility gene, and that altered neuronal migration is involved in SZ pathology.

Regulation of neural migration by the CREB/CREM transcription factors and altered Dab1 levels in CREB/CREM mutants

The family of CREB transcription factors is involved in a variety of biological processes including the development and plasticity of the nervous system. To gain further insight into the roles of CREB family members in the development of the embryonic brain, we examined the migratory phenotype of CREB1(Nescre)CREM(-/-) mutants. We found that the lack of CREB/CREM genes is accompanied by anatomical defects in specific layers of the olfactory bulb, hippocampus and cerebral cortex. These changes are associated with decreased Dab1 expression in CREB1(Nescre)CREM(-/-) mutants. Our results indicate that the lack of CREB/CREM genes, specifically in neural and glial progenitors, leads to migration abnormalities during brain development, suggesting that unidentified age-dependent factors modulate the role of CREB/CREM genes in neural development.

Molecular pathways regulating cytoskeletal organization and morphological changes in migrating neurons

Neuronal migration is a pivotal step for architectural and functional brain development. Migrating neurons exhibit various morphological changes, based on cytoskeletal organization. In addition to many genetic studies revealing the involvement of several cytoskeletal and signaling molecules in cortical neuronal migration (e.g. doublecortin, Lis1, Filamin A, cyclin-dependent kinase 5, Reelin and Dab1), cell biological studies and recently developed techniques, including in utero electroporation, have uncovered detailed functions of these molecules as well as novel molecules, such as Rho family GTPases, focal adhesion kinase, c-jun N-terminal kinase and p27(kip1). In this review, we introduce the molecular pathways underlying cortical neuronal migration and morphological changes, with particular focus on recent findings for the regulatory mechanisms of actin cytoskeleton and microtubules.

Targeted deletion of the mitogen-activated protein kinase kinase 4 gene in the nervous system causes severe brain developmental defects and premature death

The c-Jun NH2-terminal protein kinase (JNK) is a mitogen-activated protein kinase (MAPK) involved in the regulation of various physiological processes. Its activity is increased upon phosphorylation by the MAPK kinases MKK4 and MKK7. The early embryonic death of mice lacking an mkk4 or mkk7 gene has provided genetic evidence that MKK4 and MKK7 have nonredundant functions in vivo. To elucidate the physiological role of MKK4, we generated a novel mouse model in which the mkk4 gene could be specifically deleted in the brain. At birth, the mutant mice were indistinguishable from their control littermates, but they stopped growing a few days later and died prematurely, displaying severe neurological defects. Decreased JNK activity in the absence of MKK4 correlated with impaired phosphorylation of a subset of physiologically relevant JNK substrates and with altered gene expression. These defects resulted in the misalignment of the Purkinje cells in the cerebellum and delayed radial migration in the cerebral cortex. Together, our data demonstrate for the first time that MKK4 is an essential activator of JNK required for the normal development of the brain.

Reelin signals through phosphatidylinositol 3-kinase and Akt to control cortical development and through mTor to regulate dendritic growth

Reelin is an extracellular matrix protein with various functions during development and in the mature brain. It activates different signaling cascades in target cells, one of which is the phosphatidylinositol 3-kinase (PI3K) pathway, which we investigated further using pathway inhibitors and in vitro brain slice and neuronal cultures. We show that the mTor (mammalian target of rapamycin)-S6K1 (S6 kinase 1) pathway is activated by Reelin and that this depends on Dab1 (Disabled-1) phosphorylation and activation of PI3K and Akt (protein kinase B). PI3K and Akt are required for the effects of Reelin on the organization of the cortical plate, but their downstream partners mTor and glycogen synthase kinase 3beta (GSK3beta) are not. On the other hand, mTor, but not GSK3beta, mediates the effects of Reelin on the growth and branching of dendrites of hippocampal neurons. In addition, PI3K fosters radial migration of cortical neurons through the intermediate zone, an effect that is independent of Reelin and Akt.

Trekking across the Brain: The Journey of Neuronal Migration

The correct positioning of neurons during development--achieved through directed migration--is the basis for proper brain function. Several decades of research have yielded a comprehensive map illustrating the temporal and spatial events underlying neurogenesis and neuronal migration during development. The discovery of distinct migration modes and pathways has been accompanied by the identification of a large interwoven molecular network that transmits extracellular signals into the cell. Moreover, recent work has shed new light on how the cytoskeleton is regulated and coordinated at the molecular and cellular level to execute neuronal migration.

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

A mouse homologue of Strawberry Notch is transcriptionally regulated by Reelin signal

Reelin is a glycoprotein secreted by specific neuronal populations of the adult and developing nervous system of vertebrates. The morphological abnormalities in the brain of reeler, the Reelin deficient mutant mice, indicate that Reelin is essential for the brain morphogenesis. However, biochemical function of Reelin signal is not well understood. Here, we examined possible function of Reelin signal in regulation of gene expression by performing a microarray analysis. We found that expression level of a mouse homologue of Strawberry Notch (mSno1) is markedly reduced in the reeler embryos. In situ hybridization showed that mSno1 is expressed in the developing nervous system colocalizing with expression of ApoER2, a Reelin receptor. Treatment of P19 cells with Reelin protein enhanced mSno1 expression. Overexpression of ApoER2 with Reelin treatment gave a synergistic effect on mSno1 expression level. These observations suggest that Reelin signal is involved in embryonic expression of a novel vertebrate gene, mSno1.

Reelin expression and glycosylation patterns are altered in Alzheimer's disease

Reelin is a glycoprotein that is essential for the correct cytoarchitectonic organization of the developing CNS. Its function in the adult brain is less understood, although it has been proposed that Reelin is involved in signaling pathways linked to neurodegeneration. Here we analyzed Reelin expression in brains and cerebrospinal fluid (CSF) from Alzheimer's disease (AD) patients and nondemented controls. We found a 40% increase in the Reelin protein levels in the cortex of AD patients compared with controls. Similar increases were detected at the Reelin mRNA transcriptional level. This expression correlates with parallel increases in CSF but not in plasma samples. Next, we examined whether CSF Reelin levels were also altered in neurological diseases, including frontotemporal dementia, progressive supranuclear palsy, and Parkinson's disease. The Reelin 180-kDa band increased in all of the neurodegenerative disorders analyzed. Moreover, the 180-kDa Reelin levels correlated positively with Tau protein in CSF. Finally, we studied the pattern of Reelin glycosylation by using several lectins and the anti-HNK-1 antibody. Glycosylation differed in plasma and CSF. Furthermore, the pattern of Reelin lectin binding differed between the CSF of controls and in AD. Our results show that Reelin is up-regulated in the brain and CSF in several neurodegenerative diseases and that CSF and plasma Reelin have distinct cellular origins, thereby supporting that Reelin is involved in the pathogenesis of a number of neurodegenerative diseases.

Pathogenesis of migration disorders

PURPOSE OF REVIEW

Neocortical neurons have to migrate from their site of production in the periventricular germinative zone or in the ganglionic eminence towards the cortical plate. Our understanding of the underlying molecular mechanisms has advanced considerably in recent years due to the identification of genes involved in human migration disorders and experimental studies. This review will highlight some of the most recent findings in the deciphering of the molecular machinery controlling neuronal migration.

RECENT FINDINGS

Neuronal migration is a complex process which involves cytoskeletal molecules controlling the initiation of migration, leading edge extension and nucleokinesis; signalling molecules (the reelin pathway playing a central role) integrating external signals and linking them to the cytoskeleton; stop signals; and other molecular players including neurotrophins, glutamate receptors and peroxisome-derived factors. Emerging evidence supports the existence of cross-talk between these pathways.

SUMMARY

Identifying these mechanisms has shed light on typical human neuronal migration disorders such as periventricular heterotopias (disorder of migration initiation linked to filamin), type I lissencephaly (cytoskeletal abnormality linked to Lis1, a microtubule-associated protein), double cortex syndrome (cytoskeletal abnormality linked to doublecortin, a microtubule-associated protein), or lissencephaly plus cerebellar hypoplasia (reelin defect).

Reelin induces the detachment of postnatal subventricular zone cells and the expression of the Egr-1 through Erk1/2 activation

Reelin binds to very low-density lipoprotein receptor and apolipoprotein E receptor 2, thereby inducing mDab1 phosphorylation and activation of the phosphatidylinositide 3 kinase (PI3K) pathway. Here we demonstrate that Reelin activates the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) pathway, which leads to the phosphorylation of Erk1/2 proteins. The inhibition of Src family kinases (SFK) blocked Reelin-dependent Erk1/2 activation. This was also shown in neuronal cultures from mDab1-deficient mice. Although rat sarcoma viral oncogene was weakly activated upon Reelin treatment, pharmacological inhibition of the PI3K pathway blocked Reelin-dependent ERK activation, which indicates cross talk between the ERK and PI3K pathways. We show that blockade of the ERK pathway does not prevent the chain migration of neurons from the subventricular zone (SVZ) but does inhibit the Reelin-dependent detachment of migrating neurons. We also show that Reelin induces the transcription of the early growth response 1 transcription factor. Our findings demonstrate that Reelin triggers ERK signaling in an SFK/mDab1- and PI3K-dependent manner and that ERK activation is required for Reelin-dependent transcriptional activation and the detachment of neurons migrating from the SVZ.

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

Ten years following identification of Reelin as the product of the gene mutated in reeler mice, the signalling pathway activated by Reelin is being progressively unravelled with the identification of lipoprotein receptors as reelin receptors, of the Dab1 adapter and of some other proximal components in target cells. However, we are still a long way from understanding the action of this complex protein during brain development and maturation. The present review is organized in two parts. First, we summarize our present understanding of Reelin signalling. Then, we review critically some cell biological mechanisms for the action of Reelin based on recent studies on the development of the dentate gyrus, which has proved an extremely useful and tractable model system.

The control of microtubule stability in vitro and in transfected cells by MAP1B and SCG10

In neurons, the regulation of microtubules plays an important role for neurite outgrowth, axonal elongation, and growth cone steering. SCG10 family proteins are the only known neuronal proteins that have a strong destabilizing effect, are highly enriched in growth cones and are thought to play an important role during axonal elongation. MAP1B, a microtubule-stabilizing protein, is found in growth cones as well, therefore it was important to test their effect on microtubules in the presence of both proteins. We used recombinant proteins in microtubule assembly assays and in transfected COS-7 cells to analyze their combined effects in vitro and in living cells, respectively. Individually, both proteins showed their expected activities in microtubule stabilization and destruction respectively. In MAP1B/SCG10 double-transfected cells, MAP1B could not protect microtubules from SCG10-induced disassembly in most cells, in particular not in cells that contained high levels of SCG10. This suggests that SCG10 is more potent to destabilize microtubules than MAP1B to rescue them. In microtubule assembly assays, MAP1B promoted microtubule formation at a ratio of 1 MAP1B per 70 tubulin dimers while a ratio of 1 SCG10 per two tubulin dimers was needed to destroy microtubules. In addition to its known binding to tubulin dimers, SCG10 binds also to purified microtubules in growth cones of dorsal root ganglion neurons in culture. In conclusion, neuronal microtubules are regulated by antagonistic effects of MAP1B and SCG10 and a fine tuning of the balance of these proteins may be critical for the regulation of microtubule dynamics in growth cones.