Astrotactin: a novel neuronal cell surface antigen that mediates neuron-astroglial interactions in cerebellar microcultures

Research Advances in Neuroblast Migration in Traumatic Brain Injury

Neuroblasts were first derived from the adult mammalian brains in the 1990s by Reynolds et al. Since then, persistent neurogenesis in the subgranular zone (SGZ) of the hippocampus and subventricular zone (SVZ) has gradually been recognized. To date, reviews on neuroblast migration have largely investigated glial cells and molecular signaling mechanisms, while the relationship between vasculature and cell migration remains a mystery. Thus, this paper underlines the partial biological features of neuroblast migration and unravels the significance and mechanisms of the vasculature in the process to further clarify theoretically the neural repair mechanism after brain injury. Neuroblast migration presents three modes according to the characteristics of cells that act as scaffolds during the migration process: gliophilic migration, neurophilic migration, and vasophilic migration. Many signaling molecules, including brain-derived neurotrophic factor (BDNF), stromal cell-derived factor 1 (SDF-1), vascular endothelial growth factor (VEGF), and angiopoietin-1 (Ang-1), affect vasophilic migration, synergistically regulating the migration of neuroblasts to target areas along blood vessels. However, the precise role of blood vessels in the migration of neuroblasts needs to be further explored. The in-depth study of neuroblast migration will most probably provide theoretical basis and breakthrough for the clinical treatment of brain injury diseases.

Neuronal and astrocytic protein connections and associated adhesion molecules

Astrocytes are morphologically complex, with a myriad of processes which allow contact with other astrocytes, blood vessels, and neurons. Adhesion molecules expressed by these cells regulate this connectivity. Adhesion molecules are required to form and maintain functional neural circuits, but their importance and mechanisms of action, particularly in astrocyte-neuron contact, remain unresolved. Several studies of neuron-astrocyte connections have demonstrated the vital functions of adhesion molecules, including neuron-glia cell adhesion molecules, astrotactins, and protocadherins. In this review, we provide an overview and perspective of astrocyte-neuron contacts mediated by adhesion molecules in developing neural circuits and synapse formation, especially in the cerebellum. We also outline a novel mechanism of interaction between neurons and astrocytes in the tripartite synapses that has been recently found by our group.

Rare copy number variants in ASTN2 gene in patients with neurodevelopmental disorders

INTRODUCTION

In humans the normal development of cortical regions depends on the complex interactions between a number of proteins that promote the migrations of neuronal precursors from germinal zones and assembly into neuronal laminae. ASTN2 is one of the proteins implicated in such a complex process. Recently it has been observed that ASTN2 also regulates the surface expression of multiple synaptic proteins resulting in a modulation of synaptic activity. Several rare copy number variants (CNVs) in ASTN2 gene were identified in patients with neurodevelopmental disorders (NDDs) including autism spectrum disorders (ASD), attention deficit-hyperactivity disorders and intellectual disability.

METHODS

By using comparative genomic hybridization array technology, we analyzed the genomic profiles of five patients of three unrelated families with NDDs. Clinical diagnosis of ASD was established according to the Statistical Manual of Mental Disorders, Fifth Edition (APA 2013) criteria.

RESULTS

We identified new rare CNVs encompassing ASTN2 gene in three unrelated families with different clinical phenotypes of NDDs. In particular, we identified a deletion of about 70 Kb encompassing intron 19, a 186 Kb duplication encompassing the sequence between the 5'-end and the first intron of the gene and a 205 Kb deletion encompassing exons 6-11.

CONCLUSION

The CNVs reported here involve regions not usually disrupted in patients with NDDs with two of them affecting only the expression of the long isoforms. Further studies will be needed to analyze the impact of these CNVs on gene expression regulation and to better understand their impact on the protein function.

High prevalence of multilocus pathogenic variation in neurodevelopmental disorders in the Turkish population

Neurodevelopmental disorders (NDDs) are clinically and genetically heterogenous; many such disorders are secondary to perturbation in brain development and/or function. The prevalence of NDDs is > 3%, resulting in significant sociocultural and economic challenges to society. With recent advances in family-based genomics, rare-variant analyses, and further exploration of the Clan Genomics hypothesis, there has been a logarithmic explosion in neurogenetic "disease-associated genes" molecular etiology and biology of NDDs; however, the majority of NDDs remain molecularly undiagnosed. We applied genome-wide screening technologies, including exome sequencing (ES) and whole-genome sequencing (WGS), to identify the molecular etiology of 234 newly enrolled subjects and 20 previously unsolved Turkish NDD families. In 176 of the 234 studied families (75.2%), a plausible and genetically parsimonious molecular etiology was identified. Out of 176 solved families, deleterious variants were identified in 218 distinct genes, further documenting the enormous genetic heterogeneity and diverse perturbations in human biology underlying NDDs. We propose 86 candidate disease-trait-associated genes for an NDD phenotype. Importantly, on the basis of objective and internally established variant prioritization criteria, we identified 51 families (51/176 = 28.9%) with multilocus pathogenic variation (MPV), mostly driven by runs of homozygosity (ROHs) - reflecting genomic segments/haplotypes that are identical-by-descent. Furthermore, with the use of additional bioinformatic tools and expansion of ES to additional family members, we established a molecular diagnosis in 5 out of 20 families (25%) who remained undiagnosed in our previously studied NDD cohort emanating from Turkey.

Profiling NSD3-dependent neural crest gene expression reveals known and novel candidate regulatory factors

The lysine methyltransferase NSD3 is required for the expression of key neural crest transcription factors and the migration of neural crest cells. Nevertheless, a complete view of the genes dependent upon NSD3 for expression and the developmental processes impacted by NSD3 in the neural crest was lacking. We used RNA sequencing (RNA-seq) to profile transcripts differentially expressed after NSD3 knockdown in chick premigratory neural crest cells, identifying 674 genes. Gene Ontology and gene set enrichment analyses further support a requirement for NSD3 during neural crest development and show that NSD3 knockdown also upregulates ribosome biogenesis. To validate our results, we selected three genes not previously associated with neural crest development, Astrotactin 1 (Astn1), Dispatched 3 (Disp3), and Tropomyosin 1 (Tpm1). Using whole mount in situ hybridization, we show that premigratory neural crest cells express these genes and that NSD3 knockdown downregulates (Astn1 and Disp3) and upregulates (Tpm1) their expression, consistent with RNA-seq results. Altogether, this study identifies novel putative regulators of neural crest development and provides insight into the transcriptional consequences of NSD3 in the neural crest, with implications for cancer.

Origins, Development, and Compartmentation of the Granule Cells of the Cerebellum

Granule cells (GCs) are the most numerous cell type in the cerebellum and indeed, in the brain: at least 99% of all cerebellar neurons are granule cells. In this review article, we first consider the formation of the upper rhombic lip, from which all granule cell precursors arise, and the way by which the upper rhombic lip generates the external granular layer, a secondary germinal epithelium that serves to amplify the upper rhombic lip precursors. Next, we review the mechanisms by which postmitotic granule cells are generated in the external granular layer and migrate radially to settle in the granular layer. In addition, we review the evidence that far from being a homogeneous population, granule cells come in multiple phenotypes with distinct topographical distributions and consider ways in which the heterogeneity of granule cells might arise during development.

Retinal inputs signal astrocytes to recruit interneurons into visual thalamus

Inhibitory interneurons comprise a fraction of the total neurons in the visual thalamus but are essential for sharpening receptive field properties and improving contrast-gain of retinogeniculate transmission. During early development, these interneurons undergo long-range migration from germinal zones, a process regulated by the innervation of the visual thalamus by retinal ganglion cells. Here, using transcriptomic approaches, we identified a motogenic cue, fibroblast growth factor 15 (FGF15), whose expression in the visual thalamus is regulated by retinal input. Targeted deletion of functional FGF15 in mice led to a reduction in thalamic GABAergic interneurons similar to that observed in the absence of retinal input. This loss may be attributed, at least in part, to misrouting of interneurons into nonvisual thalamic nuclei. Unexpectedly, expression analysis revealed that FGF15 is generated by thalamic astrocytes and not retino-recipient neurons. Thus, these data show that retinal inputs signal through astrocytes to direct the long-range recruitment of interneurons into the visual thalamus.

A Case Study on the Keap1 Interaction with Peptide Sequence Epitopes Selected by the Peptidomic mRNA Display

Many protein-protein and peptide-protein interactions (PPIs) play key roles in the regulation of biological functions, and therefore, the modulation of PPIs has become an attractive target of new drug development. Although a number of PPIs have already been identified, over 100 000 unknown PPIs are predicted to exist. To uncover such unknown PPIs, it is important to devise a conceptually distinct method from that of currently available methods. Herein, an mRNA display by using a total RNA library derived from various human tissues, which serves as a unique method to physically isolate peptide epitopes that potentially bind to a target protein of interest, is reported. In this study, selection was performed against Kelch-like ECH-associated protein (Keap1) as a model target protein, leading to a peptide epitope originating from astrotactin-1 (ASTN1). It turned out that this ASTN1 peptide was able to interact with Keap1 more strongly than that with a known peptide derived from Nrf2; a well-known, naturally occurring Keap1 binder. This case study demonstrates the applicability of peptidomic mRNA display for the rapid exploration of consensus binding peptide motifs and the potential for the discovery of unknown PPIs with other proteins of interest.

Control of growth factor signalling by MACPF proteins

Members of the membrane attack complex/perforin-like (MACPF) protein superfamily have long captured interest because of their unique ability to assemble into large oligomeric pores on the surfaces of cells. The best characterised of these act in vertebrate immunity where they function to deliver pro-apoptotic factors or induce the cytolysis and death of targeted cells. Less appreciated, however, is that rather than causing cell death, MACPF proteins have also evolved to control cellular signalling pathways and influence developmental programmes such as pattern formation and neurogenesis. Torso-like (Tsl) from the fruit fly Drosophila, for example, functions to localise the activity of a growth factor for patterning its embryonic termini. It remains unclear whether these developmental proteins employ an attenuated form of the classical MACPF lytic pore, or if they have evolved to function via alternative mechanisms of action. In this minireview, we examine the evidence that links pore-forming MACPF proteins to the control of growth factor and cytokine signalling. We will then attempt to reconcile how the MACPF domain may have been repurposed during evolution for developmental events rather than cell killing.

Murine astrotactins 1 and 2 have a similar membrane topology and mature via endoproteolytic cleavage catalyzed by a signal peptidase

Astrotactin 1 (Astn1) and Astn2 are membrane proteins that function in glial-guided migration, receptor trafficking, and synaptic plasticity in the brain as well as in planar polarity pathways in the skin. Here we used glycosylation mapping and protease protection approaches to map the topologies of mouse Astn1 and Astn2 in rough microsomal membranes and found that Astn2 has a cleaved N-terminal signal peptide, an N-terminal domain located in the lumen of the rough microsomal membranes (topologically equivalent to the extracellular surface in cells), two transmembrane helices, and a large C-terminal lumenal domain. We also found that Astn1 has the same topology as Astn2, but we did not observe any evidence of signal peptide cleavage in Astn1. Both Astn1 and Astn2 mature through endoproteolytic cleavage in the second transmembrane helix; importantly, we identified the endoprotease responsible for the maturation of Astn1 and Astn2 as the endoplasmic reticulum signal peptidase. Differences in the degree of Astn1 and Astn2 maturation possibly contribute to the higher levels of the C-terminal domain of Astn1 detected on neuronal membranes of the central nervous system. These differences may also explain the distinct cellular functions of Astn1 and Astn2, such as in membrane adhesion, receptor trafficking, and planar polarity signaling.

Rostro-Caudal and Caudo-Rostral Migrations in the Telencephalon: Going Forward or Backward?

The generation and differentiation of an appropriate number of neurons, as well as its distribution in different parts of the brain, is crucial for the proper establishment, maintenance and plasticity of neural circuitries. Newborn neurons travel along the brain in a process known as neuronal migration, to finalize their correct position in the nervous system. Defects in neuronal migration produce abnormalities in the brain that can generate neurodevelopmental pathologies, such as autism, schizophrenia and intellectual disability. In this review, we present an overview of the developmental origin of the different telencephalic subdivisions and a description of migratory pathways taken by distinct neural populations traveling long distances before reaching their target position in the brain. In addition, we discuss some of the molecules implicated in the guidance of these migratory paths and transcription factors that contribute to the correct migration and integration of these neurons.

Fundamental Elements in Autism: From Neurogenesis and Neurite Growth to Synaptic Plasticity

Autism spectrum disorder (ASD) is a set of neurodevelopmental disorders with a high prevalence and impact on society. ASDs are characterized by deficits in both social behavior and cognitive function. There is a strong genetic basis underlying ASDs that is highly heterogeneous; however, multiple studies have highlighted the involvement of key processes, including neurogenesis, neurite growth, synaptogenesis and synaptic plasticity in the pathophysiology of neurodevelopmental disorders. In this review article, we focus on the major genes and signaling pathways implicated in ASD and discuss the cellular, molecular and functional studies that have shed light on common dysregulated pathways using in vitro, in vivo and human evidence.

Highlights

Autism spectrum disorder (ASD) has a prevalence of 1 in 68 children in the United States.

ASDs are highly heterogeneous in their genetic basis.

ASDs share common features at the cellular and molecular levels in the brain.

Most ASD genes are implicated in neurogenesis, structural maturation, synaptogenesis and function.

Seven in Absentia E3 Ubiquitin Ligases: Central Regulators of Neural Cell Fate and Neuronal Polarity

During neural development, neural precursors transition from a proliferative state within their germinal niches to a migratory state as they relocate to their final laminar positions. Transitions across these states are coupled with dynamic alterations in cellular polarity. This key feature can be seen throughout the developing vertebrate brain, in which neural stem cells give rise to multipolar or unpolarized transit-amplifying progenitors. These transit-amplifying progenitors then expand to give rise to mature neuronal lineages that become polarized as they initiate radial migration to their final laminar positions. The conventional understanding of the cellular polarity regulatory program has revolved around signaling cascades and transcriptional networks. In this review, we discuss recent discoveries concerning the role of the Siah2 ubiquitin ligase in initiating neuronal polarity during cerebellar development. Given the unique features of Siah ubiquitin ligases, we highlight some of the key substrates that play important roles in cellular polarity and propose a function for the Siah ubiquitin proteasome pathway in mediating a post-translational regulatory network to control the onset of polarization.

Repurposing a pore: highly conserved perforin-like proteins with alternative mechanisms

Pore-forming proteins play critical roles in pathogenic attack and immunological defence. The membrane attack complex/perforin (MACPF) group of homologues represents, with cholesterol-dependent cytolysins, the largest family of such proteins. In this review, we begin by describing briefly the structure of MACPF proteins, outlining their common mechanism of pore formation. We subsequently discuss some examples of MACPF proteins likely implicated in pore formation or other membrane-remodelling processes. Finally, we focus on astrotactin and bone morphogenetic protein and retinoic acid-induced neural-specific proteins, highly conserved MACPF family members involved in developmental processes, which have not been well studied to date or observed to form a pore-and which data suggest may act by alternative mechanisms.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Cleave but not leave: Astrotactin proteins in development and disease

Over the years, animal studies have identified astrotactins as important membrane proteins for glial-guided neuronal migration during central nervous system development and hair follicle polarity control during skin development. Biochemical studies have revealed intramembrane proteolysis as an important feature of astrotactins. The two fragments of astrotactins remain linked together by a disulfide bond after the proteolytic cleavage. In humans, mutations in astrotactin genes have also been linked to a wide range of diseases, including several developmental brain disorders, neurodegenerative diseases and cancer. In this review, I will summarize the current knowledge of the biological function of astrotactins in development, highlight the linkage between mutations in astrotactin genes and human disease and discuss several outstanding questions that remain unanswered. © 2017 IUBMB Life, 69(8):572-577, 2017.

Neurodevelopmental MACPFs: The vertebrate astrotactins and BRINPs

Astrotactins (ASTNs) and Bone morphogenetic protein/retinoic acid inducible neural-specific proteins (BRINPs) are two groups of Membrane Attack Complex/Perforin (MACPF) superfamily proteins that show overlapping expression in the developing and mature vertebrate nervous system. ASTN(1-2) and BRINP(1-3) genes are found at conserved loci in humans that have been implicated in neurodevelopmental disorders (NDDs). Here we review the tissue distribution and cellular localization of these proteins, and discuss recent studies that provide insight into their structure and interactions. We highlight the genetic relationships and co-expression of Brinps and Astns; and review recent knock-out mouse phenotypes that indicate a possible overlap in protein function between ASTNs and BRINPs.

Consensus Paper: Cerebellar Development

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

Structure of astrotactin-2: a conserved vertebrate-specific and perforin-like membrane protein involved in neuronal development

The vertebrate-specific proteins astrotactin-1 and 2 (ASTN-1 and ASTN-2) are integral membrane perforin-like proteins known to play critical roles in neurodevelopment, while ASTN-2 has been linked to the planar cell polarity pathway in hair cells. Genetic variations associated with them are linked to a variety of neurodevelopmental disorders and other neurological pathologies, including an advanced onset of Alzheimer's disease. Here we present the structure of the majority endosomal region of ASTN-2, showing it to consist of a unique combination of polypeptide folds: a perforin-like domain, a minimal epidermal growth factor-like module, a unique form of fibronectin type III domain and an annexin-like domain. The perforin-like domain differs from that of other members of the membrane attack complex-perforin (MACPF) protein family in ways that suggest ASTN-2 does not form pores. Structural and biophysical data show that ASTN-2 (but not ASTN-1) binds inositol triphosphates, suggesting a mechanism for membrane recognition or secondary messenger regulation of its activity. The annexin-like domain is closest in fold to repeat three of human annexin V and similarly binds calcium, and yet shares no sequence homology with it. Overall, our structure provides the first atomic-resolution description of a MACPF protein involved in development, while highlighting distinctive features of ASTN-2 responsible for its activity.

miR-sc3, a Novel MicroRNA, Promotes Schwann Cell Proliferation and Migration by Targeting Astn1

MicroRNAs (miRNAs, miRs) are small noncoding RNAs that regulate gene expression at the posttranscriptional level. We have previously identified a group of novel miRNAs in proximal sciatic nerve after sciatic nerve transection by Solexa sequencing, and miR-sc3 is a member of the group. In this study, we aimed to investigate the effects of miR-sc3 on phenotypic modulation of Schwann cells (SCs). miR-sc3 was highly expressed in the injured nerve after sciatic nerve transection. An increased and decreased expression of miR-sc3 promoted and reduced the proliferation and migration of primary SCs, respectively. miR-sc3 directly targeted astrotactin 1 (Astn1) and led to translational suppression of Astn1. There was an inverse association between the time-dependent expressions of miR-sc3 and Astn1 in proximal sciatic nerve after sciatic nerve transection. Overall, miR-sc3 affected SC proliferation and migration by targeting Astn1, thus playing the regulatory role in peripheral nerve regeneration.

Regulation of neuronal migration, an emerging topic in autism spectrum disorders

Autism spectrum disorders (ASD) encompass a group of neurodevelopmental diseases that demonstrate strong heritability, however, the inheritance is not simple and many genes have been associated with these disorders. ASD is regarded as a neurodevelopmental disorder, and abnormalities at different developmental stages are part of the disease etiology. This review provides a general background on neuronal migration during brain development and discusses recent advancements in the field connecting ASD and aberrant neuronal migration. We propose that neuronal migration impairment may be an important common pathophysiology in autism spectrum disorders (ASD). This review provides a general background on neuronal migration during brain development and discusses recent advancements in the field connecting ASD and aberrant neuronal migration.

Characterization of biological pathways associated with a 1.37 Mbp genomic region protective of hypertension in Dahl S rats

The goal of the present study was to narrow a region of chromosome 13 to only several genes and then apply unbiased statistical approaches to identify molecular networks and biological pathways relevant to blood-pressure salt sensitivity in Dahl salt-sensitive (SS) rats. The analysis of 13 overlapping subcongenic strains identified a 1.37 Mbp region on chromosome 13 that influenced the mean arterial blood pressure by at least 25 mmHg in SS rats fed a high-salt diet. DNA sequencing and analysis filled genomic gaps and provided identification of five genes in this region, Rfwd2, Fam5b, Astn1, Pappa2, and Tnr. A cross-platform normalization of transcriptome data sets obtained from our previously published Affymetrix GeneChip dataset and newly acquired RNA-seq data from renal outer medullary tissue provided 90 observations for each gene. Two Bayesian methods were used to analyze the data: 1) a linear model analysis to assess 243 biological pathways for their likelihood to discriminate blood pressure levels across experimental groups and 2) a Bayesian graphical modeling of pathways to discover genes with potential relationships to the candidate genes in this region. As none of these five genes are known to be involved in hypertension, this unbiased approach has provided useful clues to be experimentally explored. Of these five genes, Rfwd2, the gene most strongly expressed in the renal outer medulla, was notably associated with pathways that can affect blood pressure via renal transcellular Na(+) and K(+) electrochemical gradients and tubular Na(+) transport, mitochondrial TCA cycle and cell energetics, and circadian rhythms.

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

The PAR polarity complex and cerebellar granule neuron migration

Proper migration of neurons is one of the most important aspects of early brain development. After neuronal progenitors are born in their respective germinal niches, they must migrate to their final locations to form precise neural circuits. A majority of migrating neurons move by associating and disassociating with glial fibers, which serve as scaffolding for the developing brain. Cerebellar granule neurons provide a model system for examination of the mechanisms of neuronal migration in dissociated and slice culture systems; the ability to purify these cells allows migration assays to be paired with genetic, molecular, and biochemical findings. CGNs migrate in a highly polarized fashion along radial glial fibers, using a two-stroke nucleokinesis cycle. The PAR polarity complex of PARD3, PARD6, and an atypical protein kinase C (aPKC) regulate several aspects of neuronal migration. The PAR polarity complex regulates the coordinated movements of the centrosome and soma during nucleokinesis, and also the stability of the microtubule cytoskeleton during migration. PAR proteins coordinate actomyosin dynamics in the leading process of migrating neurons, which are required for migration. The PAR complex also controls the cell-cell adhesions made by migrating neurons along glial cells, and through this mechanism regulates germinal zone exit during prenatal brain development. These findings suggest that the PAR complex coordinates the movement of multiple cellular elements as neurons migrate and that further examination of PAR complex effectors will not only provide novel insights to address fundamental challenges to the field but also expand our understanding of how the PAR complex functions at the molecular level.

Disruption of the ASTN2/TRIM32 locus at 9q33.1 is a risk factor in males for autism spectrum disorders, ADHD and other neurodevelopmental phenotypes

Rare copy number variants (CNVs) disrupting ASTN2 or both ASTN2 and TRIM32 have been reported at 9q33.1 by genome-wide studies in a few individuals with neurodevelopmental disorders (NDDs). The vertebrate-specific astrotactins, ASTN2 and its paralog ASTN1, have key roles in glial-guided neuronal migration during brain development. To determine the prevalence of astrotactin mutations and delineate their associated phenotypic spectrum, we screened ASTN2/TRIM32 and ASTN1 (1q25.2) for exonic CNVs in clinical microarray data from 89 985 individuals across 10 sites, including 64 114 NDD subjects. In this clinical dataset, we identified 46 deletions and 12 duplications affecting ASTN2. Deletions of ASTN1 were much rarer. Deletions near the 3' terminus of ASTN2, which would disrupt all transcript isoforms (a subset of these deletions also included TRIM32), were significantly enriched in the NDD subjects (P = 0.002) compared with 44 085 population-based controls. Frequent phenotypes observed in individuals with such deletions include autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), speech delay, anxiety and obsessive compulsive disorder (OCD). The 3'-terminal ASTN2 deletions were significantly enriched compared with controls in males with NDDs, but not in females. Upon quantifying ASTN2 human brain RNA, we observed shorter isoforms expressed from an alternative transcription start site of recent evolutionary origin near the 3' end. Spatiotemporal expression profiling in the human brain revealed consistently high ASTN1 expression while ASTN2 expression peaked in the early embryonic neocortex and postnatal cerebellar cortex. Our findings shed new light on the role of the astrotactins in psychopathology and their interplay in human neurodevelopment.

Preliminary morphological and immunohistochemical changes in rat hippocampus following postnatal exposure to sodium arsenite

The effects of arsenic exposure during rapid brain growth period (RBGP) (postnatal period 4-11) on pyramidal neurons of cornu ammonis (specifically CA1 and CA3 regions) and granule cells of dentate gyrus (DG) of rat hippocampus were studied. Wistar rat pups, subdivided into the control (group I) and the experimental groups (group II, III, and IV), received distilled water and sodium arsenite (aqueous solution of 1.0, 1.5, and 2.0 mg/kg body weight, respectively) by intraperitoneal (i.p.) route. On postnatal day (PND) 12, the animals were sacrificed and brain tissue obtained. Paraffin sections (8 μm thick) stained with Cresyl Violet (CV) were observed for morphological and morphometric parameters. Arsenic induced programmed cell death (apoptosis) was studied using Terminal deoxyribonucleotidyl transferase mediated dUTP biotin Nick End Labeling (TUNEL) technique on the paraffin sections. Microscopy revealed decreased number and isolation of pyramidal neurons in superficial layers, misalignments of pyramidal cells in stratum pyramidale (SP) of CA1 and CA3 in experimental group III and IV, and presence of polymorphic cells in subgranular zone of ectal limb of dentate gyrus (suggestive of arsenic induced proliferation and migration of granule cells in the dentate gyrus). Morphometric assessments quantified and confirmed the microscopic findings. The mean nuclear area of pyramidal cells was increased and cell density was decreased in the CA1, CA3, and DG of experimental groups in comparison to the control group. Increase in the TUNEL positive cells in DG was observed in the experimental group IV, suggestive of increased apoptosis. These observations confirm vulnerability of pyramidal (CA1, CA3) and granule cells (DG) of hippocampus during RBGP.

Origin, lineage and function of cerebellar glia

The glial cells of the cerebellum, and particularly astrocytes and oligodendrocytes, are characterized by a remarkable phenotypic variety, in which highly peculiar morphological features are associated with specific functional features, unique among the glial cells of the entire CNS. Here, we provide a critical report about the present knowledge of the development of cerebellar glia, including lineage relationships between cerebellar neurons, astrocytes and oligodendrocytes, the origins and the genesis of the repertoire of glial types, and the processes underlying their acquisition of mature morphological and functional traits. In parallel, we describe and discuss some fundamental roles played by specific categories of glial cells during cerebellar development. In particular, we propose that Bergmann glia exerts a crucial scaffolding activity that, together with the organizing function of Purkinje cells, is necessary to achieve the normal pattern of foliation and layering of the cerebellar cortex. Moreover, we discuss some of the functional tasks of cerebellar astrocytes and oligodendrocytes that are distinctive of cerebellar glia throughout the CNS. Notably, we report about the regulation of synaptic signalling in the molecular and granular layer mediated by Bergmann glia and parenchymal astrocytes, and the functional interaction between oligodendrocyte precursor cells and neurons. On the whole, this review provides an extensive overview of the available literature and some novel insights about the origin and differentiation of the variety of cerebellar glial cells and their function in the developing and mature cerebellum.

The compartmental restriction of cerebellar interneurons

The Purkinje cells (PC's) of the cerebellar cortex are subdivided into multiple different molecular phenotypes that form an elaborate array of parasagittal stripes. This array serves as a scaffold around which afferent topography is organized. The ways in which cerebellar interneurons may be restricted by this scaffolding are less well-understood. This review begins with a brief survey of cerebellar topography. Next, it reviews the development of stripes in the cerebellum with a particular emphasis on the embryological origins of cerebellar interneurons. These data serve as a foundation to discuss the hypothesis that cerebellar compartment boundaries also restrict cerebellar interneurons, both excitatory [granule cells, unipolar brush cells (UBCs)] and inhibitory (e.g., Golgi cells, basket cells). Finally, it is proposed that the same PC scaffold that restricts afferent terminal fields to stripes may also act to organize cerebellar interneurons.

Nuclear factor one X regulates the development of multiple cellular populations in the postnatal cerebellum

Development of the cerebellum involves the coordinated proliferation, differentiation, maturation, and integration of cells from multiple neuronal and glial lineages. In rodent models, much of this occurs in the early postnatal period. However, our understanding of the molecular mechanisms that regulate this phase of cerebellar development remains incomplete. Here, we address the role of the transcription factor nuclear factor one X (NFIX), in postnatal development of the cerebellum. NFIX is expressed by progenitor cells within the external granular layer and by cerebellar granule neurons within the internal granule layer. Using NFIX⁻/⁻ mice, we demonstrate that the development of cerebellar granule neurons and Purkinje cells within the postnatal cerebellum is delayed in the absence of this transcription factor. Furthermore, the differentiation of mature glia within the cerebellum, such as Bergmann glia, is also significantly delayed in the absence of NFIX. Collectively, the expression pattern of NFIX, coupled with the delays in the differentiation of multiple cell populations of the developing cerebellum in NFIX⁻/⁻ mice, suggest a central role for NFIX in the regulation of cerebellar development, highlighting the importance of this gene for the maturation of this key structure.

ASTN1 and alcohol dependence: family-based association analysis in multiplex alcohol dependence families

A previous genome-wide linkage study of alcohol dependence (AD) in multiplex families found a suggestive linkage result for a region on Chromosome 1 near microsatellite markers D1S196 and D1S2878. The ASTN1 gene is in this region, a gene previously reported to be associated with substance abuse, bipolar disorder and schizophrenia. Using the same family data consisting of 330 individuals with phenotypic data and DNA, finer mapping of a 26 cM region centered on D1S196 was undertaken using SNPs with minor allele frequency (MAF) ≥ 0.15 and pair-wise linkage disequilibrium (LD) of r(2) < 0.8 using the HapMap CEU population. Significant FBAT P-values for SNPs within the ASTN1 gene were observed for four SNPs (rs465066, rs228008, rs6668092, and rs172917), the most significant, rs228008, within intron 8 had a P-value of 0.001. Using MQLS, which allows for inclusion of all families, we find three of these SNPs with MQLS P-values < 0.003. In addition, two additional neighboring SNPs (rs10798496 and rs6667588) showed significance at P = 0.002 and 0.03, respectively. Haplotype analysis was performed using the haplotype-based test function of FBAT for a block that included rs228008, rs6668092, and rs172917. This analysis found one block (GCG) over-transmitted and another (ATA) under-transmitted to affected offspring. Linkage analysis identified a region consistent with the association results. Family-based association analysis shows the ASTN1 gene significantly associated with alcohol dependence. The potential importance of the ASTN1 gene for AD risk may be related its role in glial-guided neuronal migration.

Cell adhesion and its endocytic regulation in cell migration during neural development and cancer metastasis

Cell migration is a crucial event for tissue organization during development, and its dysregulation leads to several diseases, including cancer. Cells exhibit various types of migration, such as single mesenchymal or amoeboid migration, collective migration and scaffold cell-dependent migration. The migration properties are partly dictated by cell adhesion and its endocytic regulation. While an epithelial-mesenchymal transition (EMT)-mediated mesenchymal cell migration requires the endocytic recycling of integrin-mediated adhesions after the disruption of cell-cell adhesions, an amoeboid migration is not dependent on any adhesions to extracellular matrix (ECM) or neighboring cells. In contrast, a collective migration is mediated by both cell-cell and cell-ECM adhesions, and a scaffold cell-dependent migration is regulated by the endocytosis and recycling of cell-cell adhesion molecules. Although some invasive carcinoma cells exhibit an EMT-mediated mesenchymal or amoeboid migration, other cancer cells are known to maintain cadherin-based cell-cell adhesions and epithelial morphology during metastasis. On the other hand, a scaffold cell-dependent migration is mainly utilized by migrating neurons in normal developing brains. This review will summarize the structures of cell adhesions, including adherens junctions and focal adhesions, and discuss the regulatory mechanisms for the dynamic behavior of cell adhesions by endocytic pathways in cell migration in physiological and pathological conditions, focusing particularly on neural development and cancer metastasis.

N-cadherin regulates radial glial fiber-dependent migration of cortical locomoting neurons

During cerebral cortical development, post-mitotic neurons exhibit a multi-step migration. The locomotion mode covers most of the neuronal migration path. Although for many decades, locomoting neurons have been known to migrate along radial glial fibers, how the cortical locomoting neurons attach to and migrate along radial glial fibers was largely unknown. We recently reported that N-cadherin is required for cortical neuronal migration in vivo. Knockdown or dominant negative inhibition of N-cadherin results in severe neuronal migration defects. Furthermore, suppression of Rab5-dependent endocytosis increases cell surface levels of N-cadherin and perturbs neuronal migration. We showed here that N-cadherin overexpression, which would mimic Rab5 suppression, weakly suppressed neuronal migration, suggesting that excess N-cadherin also disturbs neuronal migration. Interestingly, however, N-cadherin knockdown and overexpression in neurons resulted in different morphologies. While N-cadherin-overexpressing cells closely attached to the radial glial fibers similar to control or Rab5-knockdown cells, N-cadherin knockdown weakened the attachment as the average distance between the soma and radial glial fibers was significantly increased. Taken together, these findings suggest that N-cadherin controls the neuronal attachment to radial glial fibers and that N-cadherin-mediated adhesion complexes are reconstituted through Rab GTPases-dependent endocytic pathways to maintain the proper surface N-cadherin level and to promote neuronal migration.

How neurons migrate: a dynamic in-silico model of neuronal migration in the developing cortex

Background

Neuronal migration, the process by which neurons migrate from their place of origin to their final position in the brain, is a central process for normal brain development and function. Advances in experimental techniques have revealed much about many of the molecular components involved in this process. Notwithstanding these advances, how the molecular machinery works together to govern the migration process has yet to be fully understood. Here we present a computational model of neuronal migration, in which four key molecular entities, Lis1, DCX, Reelin and GABA, form a molecular program that mediates the migration process.

Results

The model simulated the dynamic migration process, consistent with in-vivo observations of morphological, cellular and population-level phenomena. Specifically, the model reproduced migration phases, cellular dynamics and population distributions that concur with experimental observations in normal neuronal development. We tested the model under reduced activity of Lis1 and DCX and found an aberrant development similar to observations in Lis1 and DCX silencing expression experiments. Analysis of the model gave rise to unforeseen insights that could guide future experimental study. Specifically: (1) the model revealed the possibility that under conditions of Lis1 reduced expression, neurons experience an oscillatory neuron-glial association prior to the multipolar stage; and (2) we hypothesized that observed morphology variations in rats and mice may be explained by a single difference in the way that Lis1 and DCX stimulate bipolar motility. From this we make the following predictions: (1) under reduced Lis1 and enhanced DCX expression, we predict a reduced bipolar migration in rats, and (2) under enhanced DCX expression in mice we predict a normal or a higher bipolar migration.

Conclusions

We present here a system-wide computational model of neuronal migration that integrates theory and data within a precise, testable framework. Our model accounts for a range of observable behaviors and affords a computational framework to study aspects of neuronal migration as a complex process that is driven by a relatively simple molecular program. Analysis of the model generated new hypotheses and yet unobserved phenomena that may guide future experimental studies. This paper thus reports a first step toward a comprehensive in-silico model of neuronal migration.

The role of Rho GTPase proteins in CNS neuronal migration

The architectonics of the mammalian brain arise from a remarkable range of directed cell migrations, which orchestrate the emergence of cortical neuronal layers and pattern brain circuitry. At different stages of cortical histogenesis, specific modes of cell motility are essential to the stepwise formation of cortical architecture. These movements range from interkinetic nuclear movements in the ventricular zone, to migrations of early-born, postmitotic polymorphic cells into the preplate, to the radial migration of precursors of cortical output neurons across the thickening cortical wall, and the vast, tangential migrations of interneurons from the basal forebrain into the emerging cortical layers. In all cases, actomyosin motors act in concert with cell adhesion receptor systems to provide the force and traction needed for forward movement. As key regulators of actin and microtubule cytoskeletons, cell polarity, and adhesion, the Rho GTPases play critical roles in CNS neuronal migration. This review will focus on the different types of migration in the developing neocortex and cerebellar cortex, and the role of the Rho GTPases, their regulators and effectors in these CNS migrations, with particular emphasis on their involvement in radial migration.

Endocytosis regulates cell soma translocation and the distribution of adhesion proteins in migrating neurons

Newborn neurons migrate from their birthplace to their final location to form a properly functioning nervous system. During these movements, young neurons must attach and subsequently detach from their substrate to facilitate migration, but little is known about the mechanisms cells use to release their attachments. We show that the machinery for clathrin-mediated endocytosis is positioned to regulate the distribution of adhesion proteins in a subcellular region just proximal to the neuronal cell body. Inhibiting clathrin or dynamin function impedes the movement of migrating neurons both in vitro and in vivo. Inhibiting dynamin function in vitro shifts the distribution of adhesion proteins to the rear of the cell. These results suggest that endocytosis may play a critical role in regulating substrate detachment to enable cell body translocation in migrating neurons.

Strategies for analyzing neuronal progenitor development and neuronal migration in the developing cerebral cortex

The emergence of functional neuronal connectivity in the developing cerebral cortex depends on 1) neural progenitor differentiation, which leads to the generation of appropriate number and types of neurons, and 2) neuronal migration, which enables the appropriate positioning of neurons so that the correct patterns of functional synaptic connectivity between neurons can emerge. In this review, we discuss 1) currently available methods to study neural progenitor development and differentiation in the developing cerebral cortex and emerging technologies in this regard, 2) assays to study the migration of descendents of progenitors (i.e., neurons) in vitro and in vivo, and 3) the use of these assays to probe the molecular control of these events in the developing brain and evaluation of gene functions disrupted in human neurodevelopmental disorders.

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.

Astn2, a novel member of the astrotactin gene family, regulates the trafficking of ASTN1 during glial-guided neuronal migration

Glial-guided neuronal migration is a key step in the development of laminar architecture of cortical regions of the mammalian brain. We previously reported that neuronal protein astrotactin (ASTN1) functions as a neuron-glial ligand during CNS glial-guided migration. Here, we identify a new Astn family member, Astn2, that is expressed at high levels in migrating, cerebellar granule neurons, along with Astn1, at developmental stages when glial-guided migration is ongoing. Biochemical and flow cytometry experiments show that ASTN2 forms a complex with ASTN1 and regulates surface expression of ASTN1. Live imaging of Venus-tagged ASTN1 in migrating cerebellar granule cells reveals the intracellular trafficking of ASTN1-Venus, with ASTN1-Venus accumulating in the forward aspect of the leading process where new sites of adhesion will form. Treatment of migrating neurons with Dynasore, a soluble noncompetitive inhibitor of Dynamin, rapidly arrests the migration of immature granule cells in a reversible manner, suggesting the critical importance of receptor trafficking to neuronal locomotion along Bergmann glial fibers in the developing cerebellum. Together, these findings suggest that ASTN2 regulates the levels of ASTN1 in the plasma membrane and that the release of neuronal adhesions to the glial fiber during neuronal locomotion involves the intracellular trafficking of ASTN1.

Myosin II motors and F-actin dynamics drive the coordinated movement of the centrosome and soma during CNS glial-guided neuronal migration

Lamination of cortical regions of the vertebrate brain depends on glial-guided neuronal migration. The conserved polarity protein Par6alpha localizes to the centrosome and coordinates forward movement of the centrosome and soma in migrating neurons. The cytoskeletal components that produce this unique form of cell polarity and their relationship to polarity signaling cascades are unknown. We show that F-actin and Myosin II motors are enriched in the neuronal leading process and that Myosin II activity is necessary for leading process actin dynamics. Inhibition of Myosin II decreased the speed of centrosome and somal movement, whereas Myosin II activation increased coordinated movement. Ectopic expression or silencing of Par6alpha inhibited Myosin II motors by decreasing Myosin light-chain phosphorylation. These findings suggest leading-process Myosin II may function to "pull" the centrosome and soma forward during glial-guided migration by a mechanism involving the conserved polarity protein Par6alpha.

Functional differentiation of a clone resembling embryonic cortical interneuron progenitors

We have generated clones (L2.3 and RG3.6) of neural progenitors with radial glial properties from rat E14.5 cortex that differentiate into astrocytes, neurons, and oligodendrocytes. Here, we describe a different clone (L2.2) that gives rise exclusively to neurons, but not to glia. Neuronal differentiation of L2.2 cells was inhibited by bone morphogenic protein 2 (BMP2) and enhanced by Sonic Hedgehog (SHH) similar to cortical interneuron progenitors. Compared with L2.3, differentiating L2.2 cells expressed significantly higher levels of mRNAs for glutamate decarboxylases (GADs), DLX transcription factors, calretinin, calbindin, neuropeptide Y (NPY), and somatostatin. Increased levels of DLX-2, GADs, and calretinin proteins were confirmed upon differentiation. L2.2 cells differentiated into neurons that fired action potentials in vitro, and their electrophysiological differentiation was accelerated and more complete when cocultured with developing astroglial cells but not with conditioned medium from these cells. The combined results suggest that clone L2.2 resembles GABAergic interneuron progenitors in the developing forebrain.

mls-2 and vab-3 Control glia development, hlh-17/Olig expression and glia-dependent neurite extension in C. elegans

Glia are essential components of nervous systems. However, genetic programs promoting glia development and regulating glia-neuron interactions have not been extensively explored. Here we describe transcriptional programs required for development and function of the C. elegans cephalic sheath (CEPsh) glia. We demonstrate ventral- and dorsal-restricted roles for the mls-2/Nkx/Hmx and vab-3/Pax6/Pax7 genes, respectively, in CEPsh glia differentiation and expression of the genes hlh-17/Olig and ptr-10/Patched-related. Using mls-2 and vab-3 mutants, as well as CEPsh glia-ablated animals, we show that CEPsh glia are important for sensory dendrite extension, axon guidance/branching within the nerve ring, and nerve ring assembly. We demonstrate that UNC-6/Netrin, expressed in ventral CEPsh glia, mediates glia-dependent axon guidance. Our results suggest possible similarities between CEPsh glia development and oligodendrocyte development in vertebrates, and demonstrate that C. elegans provides a unique environment for studying glial functions in vivo.

Cerebellar granule cell migration and the effects of alcohol

In the developing brain the majority of neurons migrate from their birthplace to their final destination. This active movement is essential for the formation of cortical layers and nuclei. The impairment of migration does not affect the viability of neurons but often results in abnormal differentiation. The proper migration of neurons requires the orchestrated activities of multiple cellular and molecular events, such as pathway selection, the activation of specific receptors and channels, and the assembly and disassembly of cytoskeletal components. The migration of neurons is very vulnerable to exposure to environmental toxins, such as alcohol. In this article, we will focus on recent developments in the migration of cerebellar granule cells. First, we will describe when, where and how granule cells migrate through different cortical layers to reach their final destination. Second, we will present how internal programs control the sequential changes in granule cell migration. Third, we will review the roles of external guidance cues and transmembrane signals in granule cell migration. Finally, we will reveal mechanisms by which alcohol exposure impairs granule cell migration.

Gliomagenesis and neural stem cells: Key role of hypoxia and concept of tumor “neo-niche”

Gliomas represent the most common primary brain tumors and the most devastating pathology of the central nervous system. Despite progress in conventional treatments, the prognosis remains dismal. Recent studies have suggested that a glioma brain tumor may arise from a "cancer stem cell". To understand this theory we summarize studies of the concepts of neural stem cell, and its specialized microenvironment, namely the niche which can regulate balanced self-renewal, differentiation and stem cell quiescence. We summarize the molecular mechanism known or postulated to be involved in the disregulation of normal stem cells features allowing them to undergo neoplasic transformation. We seek data pointing out the key role of hypoxia in normal homeostasis of stem cells and in the initiation, development and aggressiveness of gliomas. We develop the concept of tumor special microenvironment and we propose the new concept of neo-niche, surrounding the glioma, in which hypoxia could be a key factor to recruit and deregulate different stem cells for gliogenesis process. Substantial advances in treatment would come from obtaining better knowledge of molecular impairs of this disease.

Pax6-/- mice have a cell nonautonomous defect in nonradial interneuron migration

The mammalian neocortex comprises two major neuronal subtypes; interneurons derived from the ganglionic eminence (GE) and projection neurons from the cortical ventricular zone (VZ). These separate origins necessitate distinct pathways of migration. Using mouse genetics and embryonic forebrain slice culture assays, we sought to identify substrates and/or guidance molecules for nonradial cell migration (NRCM). Mice carrying a mutation in Pax6 (Sey(-/-)), a paired domain transcription factor, are reported to have increased numbers of cortical inhibitory interneurons, suggesting that Pax6 could induce inhibitors of interneuron development or alternatively play a repressive role in guiding NRCM and/or specifying interneurons. Unexpectedly, we found a cell nonautonomous reduction in the distance Sey-/- neurons migrated, reflecting a disorganized migration, with frequent changes in direction. In contrast, no difference in the number of nonradially migrating GE cells was observed in Sey-/- mice. Our data indicate that the increased numbers of interneurons observed in Sey-/- do not result from an increased rate or number of nonradially migrating cells; instead, loss of Pax6 results in the ectopic specification of interneurons in the cortical VZ. Further, our data indicate that the known axonal disorganization in Sey-/- mice contributes to the observed reduced distance of NRCM.

P2Y1 receptor switches to neurons from glia in juvenile versus neonatal rat cerebellar cortex

Background

In the CNS, several P2 receptors for extracellular nucleotides are identified on neurons and glial cells to participate to neuron-neuron, glia-glia and glia-neuron communication.

Results

In this work, we describe the cellular and subcellular presence of metabotropic P2Y₁ receptor in rat cerebellum at two distinct developmental ages, by means of immunofluorescence-confocal and electron microscopy as well as western blotting and direct membrane separation techniques. At postnatal day 21, we find that P2Y₁ receptor in addition to Purkinje neurons, is abundant on neuronal specializations identified as noradrenergic by anatomical, morphological and biochemical features. P2Y₁ receptor immunoreactivity colocalizes with dopamine β-hydroxylase, tyrosine hydroxylase, neurofilament light chain, synaptophysin and flotillin, but not with glial fibrillary acidic protein for astrocytes. P2Y₁ receptor is found enriched in membrane microdomains such as lipid rafts, in cerebellar synaptic vesicles, and is moreover visualized on synaptic varicosities by electron microscopy analysis. When examined at postnatal day 7, P2Y₁ receptor immunoreactivity is instead predominantly expressed only on Bergmann and astroglial cells, as shown by colocalization with glial fibrillary acidic protein rather then neuronal markers. At this age, we moreover identify that P2Y₁ receptor-positive Bergmann fibers wrap up doublecortin-positive granule cells stretching along them, while migrating through the cerebellar layers.

Conclusion

Membrane components including purinergic receptors are already known to mediate cellular contact and aggregation in platelets. Our results suggesting a potential role for P2Y₁ protein in cell junction/communication and development, are totally innovative for the CNS.

Bergmann glia utilize active caspase-3 for differentiation

Recently, functions associated with caspase have been modified from their well-established role in apoptosis. Although caspases are still regarded as mediators of apoptosis, some of the pro-apoptotic caspases, namely caspase-8, -14 and -3 also regulate differentiation in certain cell types, namely myelomonocytic cells, osteoblasts, skeletal muscle cells, keratinocytes, and T lymphocytes. In the central nervous system, non-apoptotic active caspase-3 expression has been located in proliferating and differentiating neuronal cells of the ventricular zone and external granular layer of the developing cerebellar cortex. We previously demonstrated that active caspase-3 expression was not limited to neuronal cells but also was located in the Bergmann glia of the postnatal cerebellum. In that study, active caspase-3 immunolabeling did not markedly colocalize with Ki67, a proliferation marker, but was present in differentiating Bergmann glia that expressed brain lipid binding protein (BLBP) and thus, by its localization, suggested a role in the differentiation of Bergmann glia. The current study addresses the function of caspase-3 in Bergmann glia development by utilizing a Bergmann glial culture preparation. Inhibition of caspase-3 activity by the peptide inhibitor, DMQD-FMK, increased the number of proliferating precursor glial cells and decreased the number of differentiating Bergmann glia, without significantly altering the non-glial active caspase-3 negative population. The transformation in the developmental state of Bergmann glia occurring after suppression of caspase-3 activity strongly suggests an involvement of this enzyme in promoting differentiation of Bergmann glia.

Glial-guided neuronal migration in P19 embryonal carcinoma stem cell aggregates

During development of the nervous system, neuronal precursors that originated in proliferative regions migrate along radial glial fibers to reach their final destination. P19 embryonal carcinoma (EC) stem cells exposed to retinoic acid (RA) differentiate into neurons, glia, and fibroblast-like cells. In this work, we induced P19 aggregates for 4 days with RA and plated them onto tissue culture dishes coated with poly-L-lysine. Several cells migrated out of and/or extended processes from the aggregates after 24 hr. Some cell processes were morphologically similar to radial glial fibers and stained for glial fibrillar acidic protein (GFAP) and nestin. Large numbers of migrating cells showed characteristics similar to those of bipolar migrating neurons and expressed the neuronal marker microtubule-associated protein 2. Furthermore, scanning electron microscopy analysis revealed an intimate association between the radial fibers and the migrating cells. Therefore, the migration of neuron-like cells on radial glia fibers in differentiated P19 aggregates resembled some of the migration models used thus far to study gliophilic neuronal migration. In addition, HPTLC analysis in this system showed the expression of 9-O-acetyl GD3, a ganglioside that has been associated with neuronal migration. Antibody perturbation assays showed that immunoblockage of 9-O-acetyl GD3 arrested neuronal migration in a reversible manner. In summary, we have characterized a new cell culture model for investigation of glial-guided neuronal migration and have shown that 9-O-acetyl GD3 ganglioside has an important role in this phenomenon.

Neuronal migration and the role of reelin during early development of the cerebral cortex

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