Should I stay or should I go? Becoming a granule cell

MMP-2 mediates Purkinje cell morphogenesis and spine development in the mouse cerebellum

Matrix metalloproteinase-2 (MMP-2) is a highly studied proteolytic enzyme, involved in many detrimental and beneficial functions throughout the body, and also active in the central nervous system (CNS). MMP-2 is profoundly expressed in the developing cerebellum and was recently reported to modulate granule cell proliferation by affecting cell cycle kinetics in cerebella of postnatal day 3 mouse pups. In this report, a two-dimensional difference gel electrophoresis proteomics study was implemented at this postnatal stage and revealed 16 differentially expressed proteins between MMP-2-deficient (MMP-2(-/-)) and wild-type cerebella. Among those, collapsin response mediator protein 1 (CRMP1) could be identified as the most significant differential protein between the two genotypes. Western blot experiments confirmed this finding and further disclosed a significant increase in phosphorylated CRMP1 expression in MMP-2(-/-) cerebella. Strikingly, subsequent immunohistochemical and microscopic analyses revealed an aberrant Purkinje cell (PC) dendritogenesis, possibly related to upregulated (phospho-) CRMP1 levels in these neonatal MMP-2(-/-) animals. Further, detailed morphometric analyses showed persistent PC morphological changes in MMP-2(-/-) mice, from the neonatal stage until adulthood. These were characterized by a reduced growth of PC somata, reduced dendritic tree sizes, and a decreased dendritic arborization. During development, the observed defects were accompanied by a temporarily disturbed parallel fiber and climbing fiber synaptic input on the PCs, while in adult MMP-2(-/-) animals, an increased PC spine density and reduced spine lengths were noted. The observed PC abnormalities might contribute to the mild defects in motor performance, i.e. balance and coordination, detected in adult MMP-2(-/-) mice. Overall, these findings indicate the importance of MMP-2 in CNS development and dendritogenesis, and highlight the importance of a correct developmental wiring for adult brain morphology and function.

Reelin and CXCL12 regulate distinct migratory behaviors during the development of the dopaminergic system

The proper functioning of the dopaminergic system requires the coordinated formation of projections extending from dopaminergic neurons in the substantia nigra (SN), ventral tegmental area (VTA) and retrorubral field to a wide array of forebrain targets including the striatum, nucleus accumbens and prefrontal cortex. The mechanisms controlling the assembly of these distinct dopaminergic cell clusters are not well understood. Here, we have investigated in detail the migratory behavior of dopaminergic neurons giving rise to either the SN or the medial VTA using genetic inducible fate mapping, ultramicroscopy, time-lapse imaging, slice culture and analysis of mouse mutants. We demonstrate that neurons destined for the SN migrate first radially and then tangentially, whereas neurons destined for the medial VTA undergo primarily radial migration. We show that tangentially migrating dopaminergic neurons express the components of the reelin signaling pathway, whereas dopaminergic neurons in their initial, radial migration phase express CXC chemokine receptor 4 (CXCR4), the receptor for the chemokine CXC motif ligand 12 (CXCL12). Perturbation of reelin signaling interferes with the speed and orientation of tangentially, but not radially, migrating dopaminergic neurons and results in severe defects in the formation of the SN. By contrast, CXCR4/CXCL12 signaling modulates the initial migration of dopaminergic neurons. With this study, we provide the first molecular and functional characterization of the distinct migratory pathways taken by dopaminergic neurons destined for SN and VTA, and uncover mechanisms that regulate different migratory behaviors of dopaminergic neurons.

Purkinje cells and Bergmann glia are primary targets of the TRα1 thyroid hormone receptor during mouse cerebellum postnatal development

Thyroid hormone is necessary for normal development of the central nervous system, as shown by the severe mental retardation syndrome affecting hypothyroid patients with low levels of active thyroid hormone. The postnatal defects observed in hypothyroid mouse cerebellum are recapitulated in mice heterozygous for a dominant-negative mutation of Thra, the gene encoding the ubiquitous TRα1 receptor. Using CRE/loxP-mediated conditional expression approach, we found that this mutation primarily alters the differentiation of Purkinje cells and Bergmann glia, two cerebellum-specific cell types. These primary defects indirectly affect cerebellum development in a global manner. Notably, the inward migration and terminal differentiation of granule cell precursors is impaired. Therefore, despite the broad distribution of its receptors, thyroid hormone targets few cell types that exert a predominant role in the network of cellular interactions that govern normal cerebellum maturation.

A role for Bicaudal-D2 in radial cerebellar granule cell migration

Bicaudal-D (BICD) belongs to an evolutionary conserved family of dynein adaptor proteins. It was first described in Drosophila as an essential factor in fly oogenesis and embryogenesis. Missense mutations in a human BICD homologue, BICD2, have been linked to a dominant mild early onset form of spinal muscular atrophy. Here we further examine the in vivo function of BICD2 in Bicd2 knockout mice. BICD2-deficient mice develop disrupted laminar organization of cerebral cortex and the cerebellum, pointing to impaired radial neuronal migration. Using astrocyte and granule cell specific inactivation of BICD2, we show that the cerebellar migration defect is entirely dependent upon BICD2 expression in Bergmann glia cells. Proteomics analysis reveals that Bicd2 mutant mice have an altered composition of extracellular matrix proteins produced by glia cells. These findings demonstrate an essential non-cell-autonomous role of BICD2 in neuronal cell migration, which might be connected to cargo trafficking pathways in glia cells.

Unconventional Neurogenic Niches and Neurogenesis Modulation by Vitamins

Although the generation of new neurons occurs in adult mammals, it has been classically described in two defined regions of the brain denominated neurogenic niches: the subventricular zone of the lateral ventricles and the subgranular zone of the dentate gyrus. In these regions, neural stem cells give rise to new neurons and glia, which functionally integrate into the existing circuits under physiological conditions. However, accumulating evidence indicates the presence of neurogenic potential in other brain regions, from which multipotent precursors can be isolated and differentiated in vitro. In some of these regions, neuron generation occurs at low levels; however, the addition of growth factors, hormones or other signaling molecules increases the proliferation and differentiation of precursor cells. In addition, vitamins, which are micronutrients necessary for normal brain development, and whose deficiency produces neurological impairments, have a regulatory effect on neural stem cells in vitro and in vivo. In the present review, we will describe the progress that has been achieved in determining the neurogenic potential in other regions, known as unconventional niches, as well as the characteristics of the neural stem cells described for each region. Finally, we will revisit the roles of commonly known vitamins as modulators of precursor cell proliferation and differentiation, and their role in the complex and tight molecular signaling that impacts these neurogenic niches.

Principal component and cluster analysis of morphological variables reveals multiple discrete sub-phenotypes in weaver mouse mutants

The present study evaluates the usefulness of the principal component analysis-based cluster analysis in the categorization of several sub-phenotypes in the weaver mutant by using several morphological parameters from the cerebellar cortex of control, heterozygous (+/wv) and homozygous (wv/wv) weaver mice. The quantified parameters were length of the cerebellar cortex, area of the external granular layer, area of the molecular layer, number of the external granular layer cells (EGL), and number of Purkinje cells (PCs). The analysis indicated that at postnatal day 8, the genotype +/wv presented three sub-phenotypes tagged as +/wv (0), +/wv (1) and +/wv (2), whereas two sub-phenotypes designated as wv (0)/wv (1) and wv (0)/wv (2) were identified in the genotype wv/wv. The number of PCs for the genotype +/wv and the number of EGL cells for the genotype wv/wv were the variables that discriminated the best among sub-phenotypes. Each one of the sub-phenotypes showed specific abnormalities in the cytoarchitecture of the cerebellar cortex as well as in the foliar pattern. In particular, the wv (0)/wv (1) and wv (0)/wv (2) sub-phenotypes had the most altered cytoarchitectonics, followed by the +/wv (2) sub-phenotype and then by the +/wv (1) one. The sub-phenotype +/wv (0) was the less affected one. Apart from reporting for the first time the coexistence of several sub-phenotypes in the weaver mutant, our approach provides a new statistical tool that can be used to assess cerebellar morphology.

Stem cells expanded from the human embryonic hindbrain stably retain regional specification and high neurogenic potency

Stem cell lines that faithfully maintain the regional identity and developmental potency of progenitors in the human brain would create new opportunities in developmental neurobiology and provide a resource for generating specialized human neurons. However, to date, neural progenitor cultures derived from the human brain have either been short-lived or exhibit restricted, predominantly glial, differentiation capacity. Pluripotent stem cells are an alternative source, but to ascertain definitively the identity and fidelity of cell types generated solely in vitro is problematic. Here, we show that hindbrain neuroepithelial stem (hbNES) cells can be derived and massively expanded from early human embryos (week 5-7, Carnegie stage 15-17). These cell lines are propagated in adherent culture in the presence of EGF and FGF2 and retain progenitor characteristics, including SOX1 expression, formation of rosette-like structures, and high neurogenic capacity. They generate GABAergic, glutamatergic and, at lower frequency, serotonergic neurons. Importantly, hbNES cells stably maintain hindbrain specification and generate upper rhombic lip derivatives on exposure to bone morphogenetic protein (BMP). When grafted into neonatal rat brain, they show potential for integration into cerebellar development and produce cerebellar granule-like cells, albeit at low frequency. hbNES cells offer a new system to study human cerebellar specification and development and to model diseases of the hindbrain. They also provide a benchmark for the production of similar long-term neuroepithelial-like stem cells (lt-NES) from pluripotent cell lines. To our knowledge, hbNES cells are the first demonstration of highly expandable neuroepithelial stem cells derived from the human embryo without genetic immortalization.

Facial motor neuron migration advances

During development, the migration of specific neuronal subtypes is required for the correct establishment of neural circuits. In mice and zebrafish, facial branchiomotor (FBM) neurons undergo a tangential migration from rhombomere 4 caudally through the hindbrain. Recent advances in the field have capitalized on genetic studies in zebrafish and mouse, and high-resolution time-lapse imaging in zebrafish. Planar cell polarity signaling has emerged as a critical conserved factor in FBM neuron migration, functioning both within the neurons and their environment. In zebrafish, migration depends on specialized 'pioneer' neurons to lead follower FBM neurons through the hindbrain, and on interactions with structural components including pre-laid axon tracts and the basement membrane. Despite fundamental conservation, species-specific differences in migration mechanisms are being uncovered.

Conditional induction of Math1 specifies embryonic stem cells to cerebellar granule neuron lineage and promotes differentiation into mature granule neurons

Directing differentiation of embryonic stem cells (ESCs) to specific neuronal subtype is critical for modeling disease pathology in vitro. An attractive means of action would be to combine regulatory differentiation factors and extrinsic inductive signals added to the culture medium. In this study, we have generated mature cerebellar granule neurons by combining a temporally controlled transient expression of Math1, a master gene in granule neuron differentiation, with inductive extrinsic factors involved in cerebellar development. Using a Tetracyclin-On transactivation system, we overexpressed Math1 at various stages of ESCs differentiation and found that the yield of progenitors was considerably increased when Math1 was induced during embryonic body stage. Math1 triggered expression of Mbh1 and Mbh2, two target genes directly involved in granule neuron precursor formation and strong expression of early cerebellar territory markers En1 and NeuroD1. Three weeks after induction, we observed a decrease in the number of glial cells and an increase in that of neurons albeit still immature. Combining Math1 induction with extrinsic factors specifically increased the number of neurons that expressed Pde1c, Zic1, and GABAα6R characteristic of mature granule neurons, formed "T-shaped" axons typical of granule neurons, and generated synaptic contacts and action potentials in vitro. Finally, in vivo implantation of Math1-induced progenitors into young adult mice resulted in cell migration and settling of newly generated neurons in the cerebellum. These results show that conditional induction of Math1 drives ESCs toward the cerebellar fate and indicate that acting on both intrinsic and extrinsic factors is a powerful means to modulate ESCs differentiation and maturation into a specific neuronal lineage.

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.

Development of cerebellar neurons and glias revealed by in utero electroporation: Golgi-like labeling of cerebellar neurons and glias

Cerebellar cortical functions rely on precisely arranged cytoarchitectures composed of several distinct types of neurons and glias. Studies have indicated that cerebellar excitatory and inhibitory neurons have distinct spatial origins, the upper rhombic lip (uRL) and ventricular zone (VZ), respectively, and that different types of neurons have different birthdates. However, the spatiotemporal relationship between uRL/VZ progenitors and their final phenotype remains poorly understood due to technical limitations. To address this issue, we performed in utero electroporation (IUE) of fluorescent protein plasmids using mouse embryos to label uRL/VZ progenitors at specific developmental stages, and observed labeled cells at maturity. To overcome any potential dilution of the plasmids caused by progenitor division, we also utilized constructs that enable permanent labeling of cells. Cerebellar neurons and glias were labeled in a Golgi-like manner enabling ready identification of labeled cells. Five types of cerebellar neurons, namely Purkinje, Golgi, Lugaro and unipolar brush cells, large-diameter deep nuclei (DN) neurons, and DN astrocytes were labeled by conventional plasmids, whereas plasmids that enable permanent labeling additionally labeled stellate, basket, and granule cells as well as three types of glias. IUE allows us to label uRL/VZ progenitors at different developmental stages. We found that the five types of neurons and DN astrocytes were labeled in an IUE stage-dependent manner, while stellate, basket, granule cells and three types of glias were labeled regardless of the IUE stage. Thus, the results indicate the IUE is an efficient method to track the development of cerebellar cells from uRL/VZ progenitors facing the ventricular lumen. They also indicate that while the generation of the five types of neurons by uRL/VZ progenitors is regulated in a time-dependent manner, the progenitor pool retains multipotency throughout embryonic development.

Ethanol neurotoxicity in the developing cerebellum: underlying mechanisms and implications

Ethanol is the main constituent of alcoholic beverages that exerts toxicity to neuronal development. Ethanol affects synaptogenesis and prevents proper brain development. In humans, synaptogenesis takes place during the third trimester of pregnancy, and in rodents this period corresponds to the initial few weeks of postnatal development. In this period neuronal maturation and differentiation begin and neuronal cells start migrating to their ultimate destinations. Although the neuronal development of all areas of the brain is affected, the cerebellum and cerebellar neurons are more susceptible to the damaging effects of ethanol. Ethanol’s harmful effects include neuronal cell death, impaired differentiation, reduction of neuronal numbers, and weakening of neuronal plasticity. Neuronal development requires many hormones and growth factors such as retinoic acid, nerve growth factors, and cytokines. These factors regulate development and differentiation of neurons by acting through various receptors and their signaling pathways. Ethanol exposure during development impairs neuronal signaling mechanisms mediated by the N-methyl-d-aspartate (NMDA) receptors, the retinoic acid receptors, and by growth factors such as brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-I), and basic fibroblast growth factor (bFGF). In combination, these ethanol effects disrupt cellular homeostasis, reduce the survival and migration of neurons, and lead to various developmental defects in the brain. Here we review the signaling mechanisms that are required for proper neuronal development, and how these processes are impaired by ethanol resulting in harmful consequences to brain development.

Cerebellar granule cells are generated postnatally in humans

How many cerebellar granule cells are generated pre- or postnatally in human is unknown. Using a rigorous design-based stereologic approach we investigated postmortem cerebella from 14 children who died between the first postnatal day (P1) and 11 months of age (M11). We found a statistically significant (p < 0.05) age-related increase in the total number of granule cells from 5.9 × 10(9) at M1 to 37.6 × 10(9) at M10/11 per cerebellar half but not in the total number of Purkinje cells (12.1 × 10(6) at M1 vs. 13.9 × 10(6) at M10/11 per cerebellar half). Accordingly, approximately 85 % of the cerebellar granule cells are generated postnatally in human, and the number of granule cells per Purkinje cell in the human cerebellum increases from 485 at M1 to 2,700 at M10/11, approximately. These data indicate that the human cerebellum has a much higher functional plasticity during the first year of life than previously thought, and may respond very sensitively to internal and external influences during this time. This has important implications for several neuropsychiatric conditions in which cerebellar involvement has been demonstrated.

Spatiotemporal expression of repulsive guidance molecules (RGMs) and their receptor neogenin in the mouse brain

Neogenin has been implicated in a variety of developmental processes such as neurogenesis, neuronal differentiation, apoptosis, migration and axon guidance. Binding of repulsive guidance molecules (RGMs) to Neogenin inhibits axon outgrowth of different neuronal populations. This effect requires Neogenin to interact with co-receptors of the uncoordinated locomotion-5 (Unc5) family to activate downstream Rho signaling. Although previous studies have reported RGM, Neogenin, and/or Unc5 expression, a systematic comparison of RGM and Neogenin expression in the developing nervous system is lacking, especially at later developmental stages. Furthermore, information on RGM and Neogenin expression at the protein level is limited. To fill this void and to gain further insight into the role of RGM-Neogenin signaling during mouse neural development, we studied the expression of RGMa, RGMb, Neogenin and Unc5A-D using in situ hybridization, immunohistochemistry and RGMa section binding. Expression patterns in the primary olfactory system, cortex, hippocampus, habenula, and cerebellum were studied in more detail. Characteristic cell layer-specific expression patterns were detected for RGMa, RGMb, Neogenin and Unc5A-D. Furthermore, strong expression of RGMa, RGMb and Neogenin protein was found on several major axon tracts such as the primary olfactory projections, anterior commissure and fasciculus retroflexus. These data not only hint at a role for RGM-Neogenin signaling during the development of different neuronal systems, but also suggest that Neogenin partners with different Unc5 family members in different systems. Overall, the results presented here will serve as a framework for further dissection of the role of RGM-Neogenin signaling during neural development.

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.

Morphology of the facial motor nuclei in a rat model of autism during early development

The development of facial nuclei in animal models of disease is poorly understood, but autism is sometimes associated with facial palsy. In the present study, to investigate migration of facial neurons and initial facial nucleus formation in an animal model of autism, rat embryos were treated with valproic acid (VPA) in utero at embryonic day (E) 9.5 and their facial nuclei were analyzed by in situ hybridization at E13.5, E14.5 and E15.5. Signals for Tbx20, which is expressed in early motor neurons, appeared near the floor plate at the level of the vestibular ganglion and extended caudolaterally, where they became ovoid in shape. This pattern of development was similar between control and VPA-exposed embryos. However, measurements of the migratory pathway and the size of the facial nuclei revealed that exposure to VPA hindered the caudal migration of neurons to the facial nuclei. Signals for cadherin 8, which is expressed in mature facial nuclei, revealed that exposure to VPA caused a significant reduction in the size of the facial nuclei. Our findings provide the first quantitative description of tangential migration and nucleus formation in the developing hindbrain in a rat model of autism.

Developmental origin of neural stem cells: the glial cell that could

Since their identification, there has been tremendous interest in adult neural stem cells, in part based upon their potential therapeutic uses in understanding and treating neurological disorders. But what's the origin of these cells in the embryo? We outline here the onset of neural specification in the vertebrate embryo and describe the molecular mechanisms regulating patterning of the central nervous system (CNS). We trace the lineage of the multipotential stem cell of the nervous system from embryonic neuroepithelial cell to adult astrocyte-like neural stem cell. As these stem cells emerge throughout development and in the adult, they appear to be predetermined to a specific neuronal or glial fate. Finally, we compare the properties of embryonic stem cell-derived neural stem cells and CNS-derived neural stem cells.

Role of the actin-binding protein profilin1 in radial migration and glial cell adhesion of granule neurons in the cerebellum

Profilins are small G-actin-binding proteins essential for cytoskeletal dynamics. Of the four mammalian profilin isoforms, profilin1 shows a broad expression pattern, profilin2 is abundant in the brain, and profilin3 and profilin4 are restricted to the testis. In vitro studies on cancer and epithelial cell lines suggested a role for profilins in cell migration and cell-cell adhesion. Genetic studies in mice revealed the importance of profilin1 in neuronal migration, while profilin2 has apparently acquired a specific function in synaptic physiology. We recently reported a mouse mutant line lacking profilin1 in the brain; animals display morphological defects that are typical for impaired neuronal migration. We found that during cerebellar development, profilin1 is specifically required for radial migration and glial cell adhesion of granule neurons. Profilin1 mutants showed cerebellar hypoplasia and aberrant organization of cerebellar cortex layers, with ectopically arranged granule neurons. In this commentary, we briefly introduce the profilin family and summarize the current knowledge on profilin activity in cell migration and adhesion. Employing cerebellar granule cells as a model, we shed some light on the mechanisms by which profilin1 may control radial migration and glial cell adhesion. Finally, a potential implication of profilin1 in human developmental neuropathies is discussed.

Neuroblast migration and P2Y(1) receptor mediated calcium signalling depend on 9-O-acetyl GD3 ganglioside

Previous studies indicated that a ganglioside 9acGD3 (9-O-acetyl GD3) antibody [the J-Ab (Jones antibody)] reduces GCP (granule cell progenitor) migration in vitro and in vivo. We here investigated, using cerebellar explants of post-natal day (P) 6 mice, the mechanism by which 9acGD3 reduces GCP migration. We found that immunoblockade of the ganglioside with the J-Ab or the lack of GD3 synthase reduced GCP in vitro migration and the frequency of Ca(2+) oscillations. Immunocytochemistry and pharmacological assays indicated that GCPs expressed P2Y(1)Rs (P2Y(1) receptors) and that deletion or blockade of these receptors decreased the migration rate of GCPs and the frequency of Ca(2+) oscillations. The reduction in P2Y(1)-mediated calcium signals seen in Jones-treated and GD3 synthase-null GCPs were paralleled by P2Y(1)R internalization. We conclude that 9acGD3 controls GCP migration by influencing P2Y(1)R cellular distribution and function.

Rp58 is essential for the growth and patterning of the cerebellum and for glutamatergic and GABAergic neuron development

Cerebellum development depends on the correct differentiation of progenitors into neurons, a process controlled by a transcriptional program that remains poorly understood. Here we show that neural-specific deletion of the BTB/POZ zinc-finger transcription factor-encoding gene Rp58 (Znf238, Zfp238) causes severe cerebellar hypoplasia and developmental failure of Purkinje neurons, Bergmann glia and granule neurons. Deletion of Rp58 in mouse embryonic Atoh1(+) progenitors leads to strong defects in growth and foliation owing to its crucial role in the differentiation of granule neurons. Analysis of the Rp58 mutant at E14.5 demonstrates that Rp58 is required for the development of both glutamatergic and GABAergic neurons. Rp58 mutants show decreased proliferation of glutamatergic progenitors at E14.5. In addition, Rp58 ablation results in a reduced number of GABAergic Pax2(+) neurons at E16.5 together with defects in the transcriptional program of ventricular zone progenitors. Our results indicate that Rp58 is essential for the growth and organization of the cerebellum and regulates the development of both GABAergic and glutamatergic neurons.

Genome-wide search reveals the existence of a limited number of thyroid hormone receptor alpha target genes in cerebellar neurons

Thyroid hormone (T3) has a major influence on cerebellum post-natal development. The major phenotypic landmark of exposure to low levels of T3 during development (hypothyroidism) in the cerebellum is the retarded inward migration of the most numerous cell type, granular neurons. In order to identify the direct genetic regulation exerted by T3 on cerebellar neurons and their precursors, we used microarray RNA hybridization to perform a time course analysis of T3 induced gene expression in primary cultures of cerebellar neuronal cell. These experiments suggest that we identified a small set of genes which are directly regulated, both in vivo and in vitro, during cerebellum post-natal development. These modest changes suggest that T3 does not acts directly on granular neurons and mainly indirectly influences the cellular interactions taking place during development.

Nos2 inactivation promotes the development of medulloblastoma in Ptch1(+/-) mice by deregulation of Gap43-dependent granule cell precursor migration

Medulloblastoma is the most common malignant brain tumor in children. A subset of medulloblastoma originates from granule cell precursors (GCPs) of the developing cerebellum and demonstrates aberrant hedgehog signaling, typically due to inactivating mutations in the receptor PTCH1, a pathomechanism recapitulated in Ptch1^+/− mice. As nitric oxide may regulate GCP proliferation and differentiation, we crossed Ptch1^+/− mice with mice lacking inducible nitric oxide synthase (Nos2) to investigate a possible influence on tumorigenesis. We observed a two-fold higher medulloblastoma rate in Ptch1^+/− Nos2^−/− mice compared to Ptch1^+/− Nos2^+/+ mice. To identify the molecular mechanisms underlying this finding, we performed gene expression profiling of medulloblastomas from both genotypes, as well as normal cerebellar tissue samples of different developmental stages and genotypes. Downregulation of hedgehog target genes was observed in postnatal cerebellum from Ptch1^+/+ Nos2^−/− mice but not from Ptch1^+/− Nos2^−/− mice. The most consistent effect of Nos2 deficiency was downregulation of growth-associated protein 43 (Gap43). Functional studies in neuronal progenitor cells demonstrated nitric oxide dependence of Gap43 expression and impaired migration upon Gap43 knock-down. Both effects were confirmed in situ by immunofluorescence analyses on tissue sections of the developing cerebellum. Finally, the number of proliferating GCPs at the cerebellar periphery was decreased in Ptch1^+/+ Nos2^−/− mice but increased in Ptch1^+/− Nos2^−/− mice relative to Ptch1^+/− Nos2^+/+ mice. Taken together, these results indicate that Nos2 deficiency promotes medulloblastoma development in Ptch1^+/− mice through retention of proliferating GCPs in the external granular layer due to reduced Gap43 expression. This study illustrates a new role of nitric oxide signaling in cerebellar development and demonstrates that the localization of pre-neoplastic cells during morphogenesis is crucial for their malignant progression.

Medulloblastoma is a common pediatric brain tumor, a subtype of which is driven by aberrant hedgehog pathway activation in cerebellar granule cell precursors. Although this tumor etiology has been intensively investigated in the well-established Ptch1^+/− mouse model, knowledge is still lacking about the molecular interactions between neoplastic transformation and other developmental processes. Nitric oxide (NO) has been reported to be involved in controlling proliferation and differentiation of these cells. Therefore, inactivation of the NO–producing enzyme Nos2 in combination with the mutated Ptch1 gene should provide insight into how developmental regulation influences pathogenesis. Here, we describe a new role for NO in developing neuronal precursors of the cerebellum facilitating physiologically accurate migration via regulation of Gap43. We further demonstrate that disturbance of these processes leads to retention of granule precursor cells to the cerebellar periphery. Together with the sustained proliferation of these cells in combined Ptch1^+/− Nos2^−/− mice, this effect results in an increased medulloblastoma incidence relative to Ptch1^+/− mice and demonstrates a new disease-promoting mechanism in this tumor entity.

SnoN signaling in proliferating cells and postmitotic neurons

The transcriptional regulator SnoN plays a fundamental role as a modulator of transforming growth factor beta (TGFβ)-induced signal transduction and biological responses. In recent years, novel functions of SnoN have been discovered in both TGFβ-dependent and TGFβ-independent settings in proliferating cells and postmitotic neurons. Accumulating evidence suggests that SnoN plays a dual role as a corepressor or coactivator of TGFβ-induced transcription. Accordingly, SnoN exerts oncogenic or tumor-suppressive effects in epithelial tissues. At the cellular level, SnoN antagonizes or mediates the ability of TGFβ to induce cell cycle arrest in a cell-type specific manner. SnoN also exerts key effects on epithelial-mesenchymal transition (EMT), with implications in cancer biology. Recent studies have expanded SnoN functions to postmitotic neurons, where SnoN orchestrates key aspects of neuronal development in the mammalian brain, from axon growth and branching to neuronal migration and positioning. In this review, we will highlight our understanding of SnoN biology at the crossroads of cancer biology and neurobiology.

Nuclear factor I and cerebellar granule neuron development: an intrinsic-extrinsic interplay

Granule neurons have a central role in cerebellar function via their synaptic interactions with other neuronal cell types both within and outside this structure. Establishment of these synaptic connections and its control is therefore essential to their function. Both intrinsic as well as environmental mechanisms are required for neuronal development and formation of neuronal circuits, and a key but poorly understood question is how these various events are coordinated and integrated in maturing neurons. In this review, we summarize recent work on the role of the Nuclear Factor I family in the transcriptional programming of cerebellar granule neuron maturation and synapse formation. In particular, we describe (1) the involvement of this family of factors in key developmental steps occurring throughout postmitotic granule neuron development, including dendrite and synapse formation and synaptic receptor expression, and (2) the mediation of these actions by critical downstream gene targets that control cell-cell interactions. These findings illustrate how Nuclear Factor I proteins and their regulons function as a “bridge” between cell-intrinsic and cell-extrinsic interactions to control multiple phases of granule neuron development.

Light stimuli control neuronal migration by altering of insulin-like growth factor 1 (IGF-1) signaling

The role of genetic inheritance in brain development has been well characterized, but little is known about the contributions of natural environmental stimuli, such as the effect of light-dark cycles, to brain development. In this study, we determined the role of light stimuli in neuronal cell migration to elucidate how environmental factors regulate brain development. We show that in early postnatal mouse cerebella, granule cell migration accelerates during light cycles and decelerates during dark cycles. Furthermore, cerebellar levels of insulin-like growth factor 1 (IGF-1) are high during light cycles and low during dark cycles. There are causal relationships between light-dark cycles, speed of granule cell migration, and cerebellar IGF-1 levels. First, changes in light-dark cycles result in corresponding changes in the fluctuations of both speed of granule cell migration and cerebellar IGF-1 levels. Second, in vitro studies indicate that exogenous IGF-1 accelerates the migration of isolated granule cells through the activation of IGF-1 receptors. Third, in vivo studies reveal that inhibiting the IGF-1 receptors decelerates granule cell migration during light cycles (high IGF-1 levels) but does not alter migration during dark cycles (low IGF-1 levels). In contrast, stimulating the IGF-1 receptors accelerates granule cell migration during dark cycles (low IGF-1 levels) but does not alter migration during light cycles (high IGF-1 levels). These results suggest that during early postnatal development light stimuli control granule cell migration by altering the activity of IGF-1 receptors through modification of cerebellar IGF-1 levels.

Profilin1 is required for glial cell adhesion and radial migration of cerebellar granule neurons

Cerebellar granule neurons (CGNs) exploit Bergmann glia (BG) fibres for radial migration, and cell-cell contacts have a pivotal role in this process. Nevertheless, little is known about the mechanisms that control CGN-BG interaction. Here we demonstrate that the actin-binding protein profilin1 is essential for CGN-glial cell adhesion and radial migration. Profilin1 ablation from mouse brains leads to a cerebellar hypoplasia, aberrant organization of cerebellar cortex layers and ectopic CGNs. Conversely, neuronal progenitor proliferation, tangential migration of neurons and BG morphology appear to be independent of profilin1. Our mouse data and the mapping of developmental neuropathies to the chromosomal region of PFN1 suggest a similar function for profilin1 in humans.

VEGF modulates NMDA receptors activity in cerebellar granule cells through Src-family kinases before synapse formation

NMDA type glutamate receptors (NMDARs) are best known for their role in synaptogenesis and synaptic plasticity. Much less is known about their developmental role before neurons form synapses. We report here that VEGF, which promotes migration of granule cells (GCs) during postnatal cerebellar development, enhances NMDAR-mediated currents and Ca(2+) influx in immature GCs before synapse formation. The VEGF receptor Flk1 forms a complex with the NMDAR subunits NR1 and NR2B. In response to VEGF, the number of Flk1/NR2B coclusters on the cell surface increases. Stimulation of Flk1 by VEGF activates Src-family kinases, which increases tyrosine phosphorylation of NR2B. Inhibition of Src-family kinases abolishes the VEGF-dependent NR2B phosphorylation and amplification of NMDAR-mediated currents and Ca(2+) influx in GCs. These findings identify VEGF as a modulator of NMDARs before synapse formation and highlight a link between an activity-independent neurovascular guidance cue (VEGF) and an activity-regulated neurotransmitter receptor (NMDAR).

Neural circuit formation in the cerebellum is controlled by cell adhesion molecules of the Contactin family

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) have been implicated in neural circuit formation in both the peripheral and the central nervous system. Several recent studies highlight a role of the Contactin group of IgSF CAMs in cerebellar development, in particular in the development of granule cells. Granule cells are the most numerous type of neurons in the nervous system and by forming a secondary proliferative zone in the cerebellum they provide an exception to the rule that neuronal precursors proliferate in the ventricular zone. Granule cells express Contactin-2, Contactin-1, and Contactin-6 in a sequential manner. Contactins are required for axon guidance, fasciculation, and synaptogenesis, and thus affect multiple steps in neural circuit formation in the developing cerebellum.

Alcohol exposure decreases CREB binding protein expression and histone acetylation in the developing cerebellum

Background

Fetal alcohol exposure affects 1 in 100 children making it the leading cause of mental retardation in the US. It has long been known that alcohol affects cerebellum development and function. However, the underlying molecular mechanism is unclear.

Methodology/Principal Findings

We demonstrate that CREB binding protein (CBP) is widely expressed in granule and Purkinje neurons of the developing cerebellar cortex of naïve rats. We also show that exposure to ethanol during the 3^rd trimester-equivalent of human pregnancy reduces CBP levels. CBP is a histone acetyltransferase, a component of the epigenetic mechanism controlling neuronal gene expression. We further demonstrate that the acetylation of both histone H3 and H4 is reduced in the cerebellum of ethanol- treated rats.

Conclusions/Significance

These findings indicate that ethanol exposure decreases the expression and function of CBP in the developing cerebellum. This effect of ethanol may be responsible for the motor coordination deficits that characterize fetal alcohol spectrum disorders.

Genome-wide profiling of G protein-coupled receptors in cerebellar granule neurons using high-throughput, real-time PCR

Background

G protein-coupled receptors (GPCRs) are major players in cell communication, regulate a whole range of physiological functions during development and throughout adult life, are affected in numerous pathological situations, and constitute so far the largest class of drugable targets for human diseases. The corresponding genes are usually expressed at low levels, making accurate, genome-wide quantification of their expression levels a challenging task using microarrays.

Results

We first draw an inventory of all endo-GPCRs encoded in the murine genome. To profile GPCRs genome-wide accurately, sensitively, comprehensively, and cost-effectively, we designed and validated a collection of primers that we used in quantitative RT-PCR experiments. We experimentally validated a statistical approach to analyze genome-wide, real-time PCR data. To illustrate the usefulness of this approach, we determined the repertoire of GPCRs expressed in cerebellar granule neurons and neuroblasts during postnatal development.

Conclusions

We identified tens of GPCRs that were not detected previously in this cell type; these GPCRs represent novel candidate players in the development and survival of cerebellar granule neurons. The sequences of primers used in this study are freely available to those interested in quantifying GPCR expression comprehensively.

Nav2 hypomorphic mutant mice are ataxic and exhibit abnormalities in cerebellar development

Development of the cerebellum involves a coordinated program of neuronal process outgrowth and migration resulting in a foliated structure that plays a key role in motor function. Neuron navigator 2 (Nav2) is a cytoskeletal-interacting protein that functions in neurite outgrowth and axonal elongation. Herein we show that hypomorphic mutant mice lacking the full-length Nav2 transcript exhibit ataxia and defects in cerebellar development. At embryonic day (E)17.5, the mutant cerebellum is reduced in size and exhibits defects in vermal foliation. Reduction in cell proliferation at early times (E12.5 and E14.5) may contribute to this size reduction. The full-length Nav2 transcript is expressed in the premigratory zone of the external granule layer (EGL). Granule cells in the germinal zone of the EGL appear to proliferate normally, however, due to the reduction in cerebellar circumference there are fewer total BrdU-labeled granule cells in the mutants, and these fail to migrate normally toward the interior of the cerebellum. In Nav2 hypomorphs, fewer granule cells migrate out of cerebellar EGL explants and neurite outgrowth from both explants and isolated external granule cell cultures is reduced. This suggests that the formation of parallel axon fibers and neuronal migration is disrupted in Nav2 mutants. This work supports an essential role for full-length Nav2 in cerebellar development, including axonal elongation and migration of the EGL neurons.

The long adventurous journey of rhombic lip cells in jawed vertebrates: a comparative developmental analysis

This review summarizes vertebrate rhombic lip and early cerebellar development covering classic approaches up to modern developmental genetics which identifies the relevant differential gene expression domains and their progeny. Most of this information is derived from amniotes. However, progress in anamniotes, particularly in the zebrafish, has recently been made. The current picture suggests that rhombic lip and cerebellar development in jawed vertebrates (gnathostomes) share many characteristics. Regarding cerebellar development, these include a ptf1a expressing ventral cerebellar proliferation (VCP) giving rise to Purkinje cells and other inhibitory cerebellar cell types, and an atoh1 expressing upper rhombic lip giving rise to an external granular layer (EGL, i.e., excitatory granule cells) and an early ventral migration into the anterior rhombencephalon (cholinergic nuclei). As for the lower rhombic lip (LRL), gnathostome commonalities likely include the formation of precerebellar nuclei (mossy fiber origins) and partially primary auditory nuclei (likely convergently evolved) from the atoh1 expressing dorsal zone. The fate of the ptf1a expressing ventral LRL zone which gives rise to (excitatory cells of) the inferior olive (climbing fiber origin) and (inhibitory cells of ) cochlear nuclei in amniotes, has not been determined in anamniotes. Special for the zebrafish in comparison to amniotes is the predominant origin of anamniote excitatory deep cerebellar nuclei homologs (i.e., eurydendroid cells) from ptf1a expressing VCP cells, the sequential activity of various atoh1 paralogs and the incomplete coverage of the subpial cerebellar plate with proliferative EGL cells. Nevertheless, the conclusion that a rhombic lip and its major derivatives evolved with gnathostome vertebrates only and are thus not an ancestral craniate character complex is supported by the absence of a cerebellum (and likely absence of its afferent and efferent nuclei) in jawless fishes

In vivo Atoh1 targetome reveals how a proneural transcription factor regulates cerebellar development

The proneural, basic helix-loop-helix transcription factor Atoh1 governs the development of numerous key neuronal subtypes, such as cerebellar granule and brainstem neurons, inner ear hair cells, and several neurons of the proprioceptive system, as well as diverse nonneuronal cell types, such as Merkel cells and intestinal secretory lineages. However, the mere handful of targets that have been identified barely begin to account for Atoh1's astonishing range of functions, which also encompasses seemingly paradoxical activities, such as promoting cell proliferation and medulloblastoma formation in the cerebellum and inducing cell cycle exit and suppressing tumorigenesis in the intestine. We used a multipronged approach to create a comprehensive, unbiased list of over 600 direct Atoh1 target genes in the postnatal cerebellum. We found that Atoh1 binds to a 10 nucleotide motif (AtEAM) to directly regulate genes involved in migration, cell adhesion, metabolism, and other previously unsuspected functions. This study expands current thinking about the transcriptional activities driving neuronal differentiation and provides a framework for further neurodevelopmental studies.

Semaphorin 4C and 4G are ligands of Plexin-B2 required in cerebellar development

Semaphorins and Plexins are cognate ligand-receptor families that regulate important steps during nervous system development. The Plexin-B2 receptor is critically involved in neural tube closure and cerebellar granule cell development, however, its specific ligands have only been suggested by in vitro studies. Here, we show by in vivo and in vitro analyses that the two Semaphorin-4 family members Sema4C and Sema4G are likely to be in vivo ligands of Plexin-B2. The Sema4C and Sema4G genes are expressed in the developing cerebellar cortex, and Sema4C and Sema4G proteins specifically bind to Plexin-B2 expressing cerebellar granule cells. To further elucidate their in vivo function, we have generated and analyzed Sema4C and Sema4G knockout mouse mutants. Like Plexin-B2-/- mutants, Sema4C-/- mutants reveal exencephaly and subsequent neonatal lethality with partial penetrance. Sema4C-/- mutants that bypass exencephaly are viable and fertile, but display distinctive defects of the cerebellar granule cell layer, including gaps in rostral lobules, fusions of caudal lobules, and ectopic granule cells in the molecular layer. In addition to neuronal defects, we observed in Sema4C-/- mutants also ventral skin pigmentation defects that are similar to those found in Plexin-B2-/- mutants. The Sema4G gene deletion causes no overt phenotype by itself, but combined deletion of Sema4C and Sema4G revealed an enhanced cerebellar phenotype. However, Sema4C/Sema4G double mutants showed overall less severe cerebellar phenotypes than Plexin-B2-/- mutants, indicating that further ligands of Plexin-B2 exist. In explant cultures of the developing cerebellar cortex, Sema4C promoted migration of cerebellar granule cell precursors in a Plexin-B2-dependent manner, supporting the model that a reduced migration rate of granule cell precursors is the basis for the cerebellar defects of Sema4C-/- and Sema4C/Sema4G mutants.

F3/contactin and TAG1 play antagonistic roles in the regulation of sonic hedgehog-induced cerebellar granule neuron progenitor proliferation

Modulation of the sonic hedgehog (SHH) pathway is a crucial factor in cerebellar morphogenesis. Stimulation of granule neuron progenitor (GNP) proliferation is a central function of SHH signalling, but how this is controlled locally is not understood. We show that two sequentially expressed members of the contactin (CNTN) family of adhesion molecules, TAG1 and F3, act antagonistically to control SHH-induced proliferation: F3 suppresses SHH-induced GNP proliferation and induces differentiation, whereas TAG1 antagonises F3. Production of GNPs in TAG1-null mice is delayed and reduced. F3 and TAG1 colocalise on GNPs with the related L1-like adhesion molecule NrCAM, and F3 fails to suppress the SHH-induced proliferation of NrCAM-deficient GNPs. We show that F3 and SHH both primarily affect a group of intermediate GNPs (IPs), which, though actively dividing, also express molecules associated with differentiation, including β-tubulin III (TuJ1) and TAG1. In vivo, intermediate progenitors form a discrete layer in the middle of the external germinal layer (mEGL), while F3 becomes expressed on the axons of postmitotic granule neurons as they leave the inner EGL (iEGL). We propose, therefore, that F3 acts as a localised signal in the iEGL that induces SHH-stimulated cells in the overlying mEGL to exit cell cycle and differentiate. By contrast, expression of TAG1 on GNPs antagonises this signal in the mEGL, preventing premature differentiation and sustaining GNP expansion in a paracrine fashion. Together, these findings indicate that CNTN and L1-like proteins play a significant role in modulating SHH-induced neuronal precursor proliferation.

Matrix-binding vascular endothelial growth factor (VEGF) isoforms guide granule cell migration in the cerebellum via VEGF receptor Flk1

Vascular endothelial growth factor (VEGF) regulates angiogenesis, but also has important, yet poorly characterized roles in neuronal wiring. Using several genetic and in vitro approaches, we discovered a novel role for VEGF in the control of cerebellar granule cell (GC) migration from the external granule cell layer (EGL) toward the Purkinje cell layer (PCL). GCs express the VEGF receptor Flk1, and are chemoattracted by VEGF, whose levels are higher in the PCL than EGL. Lowering VEGF levels in mice in vivo or ectopic VEGF expression in the EGL ex vivo perturbs GC migration. Using GC-specific Flk1 knock-out mice, we provide for the first time in vivo evidence for a direct chemoattractive effect of VEGF on neurons via Flk1 signaling. Finally, using knock-in mice expressing single VEGF isoforms, we show that pericellular deposition of matrix-bound VEGF isoforms around PC dendrites is necessary for proper GC migration in vivo. These findings identify a previously unknown role for VEGF in neuronal migration.

Calmodulin-kinases regulate basal and estrogen stimulated medulloblastoma migration via Rac1

Medulloblastoma is a highly prevalent pediatric central nervous system malignancy originating in the cerebellum, with a strong propensity for metastatic migration to the leptomeninges, which greatly increases mortality. While numerous investigations are focused on the molecular mechanisms of medulloblastoma histogenesis, the signaling pathways regulating migration are still poorly understood. Medulloblastoma likely arises from aberrant proliferative signaling in cerebellar granule precursor cells during development, and estrogen is a morphogen that promotes medulloblastoma cell migration. It has been previously shown that the calcium/calmodulin activated kinase kinase (CaMKK) pathway promotes cerebellar granule precursor migration and differentiation during normal cerebellar development via CaMKIV. Here we investigate the regulatory role of the CaMKK pathway in migration of the human medulloblastoma DAOY and cerebellar granule cells. Using pharmacological inhibitors and dominant negative approaches, we demonstrate that the CaMKK/CaMKI cascade regulates basal medulloblastoma cell migration via Rac1, in part by activation of the RacGEF, βPIX. Additionally, pharmacological inhibition of CaMKK blocks both the estrogen induced Rac1 activation and medulloblastoma migration. The CaMKK signaling module described here is one of the first reported calcium regulated pathways that modulates medulloblastoma migration. Since tumor dissemination requires cell migration to ectopic sites, this CaMKK pathway may be a putative therapeutic target to limit medulloblastoma metastasis.