Filopodia and growth cones in the vertically migrating granule cells of the postnatal mouse cerebellum

Models of Purkinje cell dendritic tree selection during early cerebellar development

We investigate the relationship between primary dendrite selection of Purkinje cells and migration of their presynaptic partner granule cells during early cerebellar development. During postnatal development, each Purkinje cell grows more than three dendritic trees, from which a primary tree is selected for development, whereas the others completely retract. Experimental studies suggest that this selection process is coordinated by physical and synaptic interactions with granule cells, which undergo a massive migration at the same time. However, technical limitations hinder continuous experimental observation of multiple cell populations. To explore possible mechanisms underlying this selection process, we constructed a computational model using a new computational framework, NeuroDevSim. The study presents the first computational model that simultaneously simulates Purkinje cell growth and the dynamics of granule cell migrations during the first two postnatal weeks, allowing exploration of the role of physical and synaptic interactions upon dendritic selection. The model suggests that interaction with parallel fibers is important to establish the distinct planar morphology of Purkinje cell dendrites. Specific rules to select which dendritic trees to keep or retract result in larger winner trees with more synaptic contacts than using random selection. A rule based on afferent synaptic activity was less effective than rules based on dendritic size or numbers of synapses.

Cellular and Subcellular Contact Guidance on Microfabricated Substrates

Topography of the extracellular environment is now recognized as a major biophysical regulator of cell behavior and function. The study of the influence of patterned substrates on cells, named contact guidance, has greatly benefited from the development of micro and nano-fabrication techniques, allowing the emergence of increasingly diverse and elaborate engineered platforms. The purpose of this review is to provide a comprehensive view of the process of contact guidance from cellular to subcellular scales. We first classify and illustrate the large diversity of topographies reported in the literature by focusing on generic cellular responses to diverse topographical cues. Subsequently, and in a complementary fashion, we adopt the opposite approach and highlight cell type-specific responses to classically used topographies (arrays of pillars or grooves). Finally, we discuss recent advances on the key subcellular and molecular players involved in topographical sensing. Throughout the review, we focus particularly on neuronal cells, whose unique morphology and behavior have inspired a large body of studies in the field of topographical sensing and revealed fascinating cellular mechanisms. We conclude by using the current understanding of the cell-topography interactions at different scales as a springboard for identifying future challenges in the field of contact guidance.

Impaired neurogenesis and associated gliosis in mouse brain with PEX13 deficiency

Zellweger syndrome (ZS), a neonatal lethal disorder arising from defective peroxisome biogenesis, features profound neuroanatomical abnormalities and brain dysfunction. Here we used mice with brain-restricted inactivation of the peroxisome biogenesis gene PEX13 to model the pathophysiological features of ZS, and determine the impact of peroxisome dysfunction on neurogenesis and cell maturation in ZS. In the embryonic and postnatal PEX13 mutant brain, we demonstrate key regions with altered brain anatomy, including enlarged lateral ventricles and aberrant cortical, hippocampal and hypothalamic organization. To characterize the underlying mechanisms, we show a significant reduction in proliferation, migration, differentiation, and maturation of neural progenitors in embryonic E12.5 through to P3 animals. An increasing reactive gliosis in the PEX13 mutant brain started at E14.5 in association with the pathology. Together with impaired neurogenesis and associated gliosis, our data demonstrate increased cell death contributing to the hallmark brain anatomy of ZS. We provide unique data where impaired neurogenesis and migration are shown as critical events underlying the neuropathology and altered brain function of mice with peroxisome deficiency.

Navigation rules for vessels and neurons: cooperative signaling between VEGF and neural guidance cues

Many organs, such as lungs, nerves, blood and lymphatic vessels, consist of complex networks that carry flows of information, gases, and nutrients within the body. The morphogenetic patterning that generates these organs involves the coordinated action of developmental signaling cues that guide migration of specialized cells. Precision guidance of endothelial tip cells by vascular endothelial growth factors (VEGFs) is well established, and several families of neural guidance molecules have been identified to exert guidance function in both the nervous and the vascular systems. This review discusses recent advances in VEGF research, focusing on the emerging role of neural guidance molecules as key regulators of VEGF function during vascular development and on the novel role of VEGFs in neural cell migration and nerve wiring.

Forward transport of proteins in the plasma membrane of migrating cerebellar granule cells

Directional flow of membrane components has been detected at the leading front of fibroblasts and the growth cone of neuronal processes, but whether there exists global directional flow of plasma membrane components over the entire migrating neuron remains largely unknown. By analyzing the trajectories of antibody-coated single quantum dots (QDs) bound to two membrane proteins, overexpressed myc-tagged synaptic vesicle-associated membrane protein VAMP2 and endogenous neurotrophin receptor TrkB, we found that these two proteins exhibited net forward transport, which is superimposed upon Brownian motion, in both leading and trailing processes of migrating cerebellar granule cells in culture. Furthermore, no net directional transport of membrane proteins was observed in nonmigrating cells with either growing or stalling leading processes. Analysis of the correlation of motion direction between two QDs on the same process in migrating neurons also showed a higher frequency of correlated forward than rearward movements. Such correlated QD movements were markedly reduced in the presence of myosin II inhibitor blebbistatin,suggesting the involvement of myosin II-dependent active transport processes. Thus, a net forward transport of plasma membrane proteins exists in the leading and trailing processes of migrating neurons, in line with the translocation of the soma.

Inhibition of cerebellar granule cell turning by alcohol

Ectopic neurons are often found in the brains of fetal alcohol spectrum disorders (FASD) and fetal alcohol syndrome (FAS) patients, suggesting that alcohol exposure impairs neuronal cell migration. Although it has been reported that alcohol decreases the speed of neuronal cell migration, little is known about whether alcohol also affects the turning of neurons. Here we show that ethanol exposure inhibits the turning of cerebellar granule cells in vivo and in vitro. First, in vivo studies using P10 mice demonstrated that a single intraperitoneal injection of ethanol not only reduces the number of turning granule cells but also alters the mode of turning at the EGL-ML border of the cerebellum. Second, in vitro analysis using microexplant cultures of P0-P3 mouse cerebella revealed that ethanol directly reduces the frequency of spontaneous granule cell turning in a dose-dependent manner. Third, the action of ethanol on the frequency of granule cell turning was significantly ameliorated by stimulating Ca(2+) and cGMP signaling or by inhibiting cAMP signaling. Taken together, these results indicate that ethanol affects the frequency and mode of cerebellar granule cell turning through alteration of the Ca(2+) and cyclic nucleotide signaling pathways, suggesting that the abnormal allocation of neurons found in the brains of FASD and FSA patients results, at least in part, from impaired turning of immature neurons by alcohol.

Topography, cell response, and nerve regeneration

In the body, cells encounter a complex milieu of signals, including topographical cues, in the form of the physical features of their surrounding environment. Imposed topography can affect cells on surfaces by promoting adhesion, spreading, alignment, morphological changes, and changes in gene expression. Neural response to topography is complex, and it depends on the dimensions and shapes of physical features. Looking toward repair of nerve injuries, strategies are being explored to engineer guidance conduits with precise surface topographies. How neurons and other cell types sense and interpret topography remains to be fully elucidated. Studies reviewed here include those of topography on cellular organization and function as well as potential cellular mechanisms of response.

Leading tip drives soma translocation via forward F-actin flow during neuronal migration

Neuronal migration involves coordinated extension of the leading process and translocation of the soma, but the relative contribution of different subcellular regions, including the leading process and cell rear, in driving soma translocation remains unclear. By local manipulation of cytoskeletal components in restricted regions of cultured neurons, we examined the molecular machinery underlying the generation of traction force for soma translocation during neuronal migration. In actively migrating cerebellar granule cells in culture, a growth cone (GC)-like structure at the leading tip exhibits high dynamics, and severing the tip or disrupting its dynamics suppressed soma translocation within minutes. Soma translocation was also suppressed by local disruption of F-actin along the leading process but not at the soma, whereas disrupting microtubules along the leading process or at the soma accelerated soma translocation. Fluorescent speckle microscopy using GFP-alpha-actinin showed that a forward F-actin flow along the leading process correlated with and was required for soma translocation, and such F-actin flow depended on myosin II activity. In migrating neurons, myosin II activity was high at the leading tip but low at the soma, and increasing or decreasing this front-to-rear difference accelerated or impeded soma advance. Thus, the tip of the leading process actively pulls the soma forward during neuronal migration through a myosin II-dependent forward F-actin flow along the leading process.

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

Cerebellar granule cells undergo profound and rapid morphological modifications during development while they migrate from their birthplace at the surface of the cerebellar cortex to its deepest layer. Post-mitotic granule cells extend bipolar axons and sequentially use the two main modes of migration, tangential and radial, to reach their final destinations. Recent studies show that protein degradation involving key cell-cycle regulators controls granule cell axon extension. The use of knockout mice deficient in different axon-guidance molecules combined with cutting-edge imaging methods has started to shed light on the molecular mechanisms that trigger granule cell migration. These studies suggest that a major reorganization of the cytoskeleton occurs as granule cells switch from tangential to radial migration.

Autonomous turning of cerebellar granule cells in vitro by intrinsic programs

External guidance cues play a role in controlling neuronal cell turning in the developing brain, but little is known about whether intrinsic programs are also involved in controlling the turning. In this study, we examined whether granule cells undergo autonomous changes in the direction of migration in the microexplant cultures of the early postnatal mouse cerebellum. We found that granule cells exhibit spontaneous and periodical turning without cell-cell contact and in the absence of external guidance cues. The frequency of turning was increased by stimulating the Ca(2+) influx and the internal Ca(2+) release, or inhibiting the cAMP signaling pathway, while the frequency was reduced by inhibiting the Ca(2+) influx. Granule cell turning in vitro was classified into four distinct modes, which were characterized by the morphological changes in the leading process and the trailing process, such as bifurcating, turning, withdrawing, and changing the polarity. The occurrence of the 1st and 2nd modes of turning was differentially affected by altering the Ca(2+) and cAMP signaling pathways. Collectively, the results demonstrate that intrinsic programs regulate the autonomous turning of cerebellar granule cells in vitro. Furthermore, the results suggest that extrinsic signals play a role as essential modulators of intrinsic programs.

The docking protein Cas links tyrosine phosphorylation signaling to elongation of cerebellar granule cell axons

Crk-associated substrate (Cas) is a tyrosine-phosphorylated docking protein that is indispensable for the regulation of the actin cytoskeletal organization and cell migration in fibroblasts. The function of Cas in neurons, however, is poorly understood. Here we report that Cas is dominantly enriched in the brain, especially the cerebellum, of postnatal mice. During cerebellar development, Cas is highly tyrosine phosphorylated and is concentrated in the neurites and growth cones of granule cells. Cas coimmunoprecipitates with Src family protein tyrosine kinases, Crk, and cell adhesion molecules and colocalizes with these proteins in granule cells. The axon extension of granule cells is inhibited by either RNA interference knockdown of Cas or overexpression of the Cas mutant lacking the YDxP motifs, which are tyrosine phosphorylated and thereby interact with Crk. These findings demonstrate that Cas acts as a key scaffold that links the proteins associated with tyrosine phosphorylation signaling pathways to the granule cell axon elongation.

Aligned neurite bundles of granule cells regulate orientation of Purkinje cell dendrites by perpendicular contact guidance in two-dimensional and three-dimensional mouse cerebellar cultures

To identify structures that determine the 90 degree orientation of thin espalier dendritic trees of Purkinje cells with respect to parallel fibers (axonal neurite bundles of granule cells) in the cerebellar cortex, we designed five types of two-dimensional and three-dimensional cell and tissue cultures of cerebella from postnatal mice and analyzed the orientation of Purkinje cell dendrites with respect to neurite bundles and astrocyte fibers by immunofluorescence double or triple staining. We cultured dissociated cerebellar cells on micropatterned substrates and preformed neurite bundles of a microexplant culture two-dimensionally and in matrix gels three-dimensionally. Dendrites, but not axons, of Purkinje cells extended toward the neurites of granule cells and oriented at right angles two-dimensionally to aligned neurite bundles in the three cultures. In a more organized explant proper of the microexplant culture, Purkinje cell dendrites extended toward thin aligned neurite bundles not only consistently at right angles but also two-dimensionally. However, in the "organotypic microexplant culture," in which three-dimensionally aligned thick neurite bundles mimicking parallel fibers were produced, Purkinje cell dendrites often oriented perpendicular to the thick bundles three-dimensionally. Astrocytes were abundant in all cultures, and there was no definite correlation between the presence of and orientation to Purkinje cell dendrites, although their fibers were frequently associated in parallel with dendrites in the organotypic microexplant culture. Therefore, Purkinje cells may grow their dendrites to the newly produced neurite bundles of parallel fibers in the cerebellar cortex and be oriented at right angles three-dimensionally mainly via "perpendicular contact guidance."

Cerebellar granule cell and Bergmann glial cell maturation in the rat is disrupted by pre- and post-natal exposure to moderate levels of ethanol

Migration of the external granular layer cells in the cerebellum of rats was delayed after exposure to moderate levels of ethanol during a pre-gestational period, through gestation and lactation until weaning. After ethanol withdrawal, cell soma and dendrites were observed to be larger in granule cells. Likewise, Bergmann glia showed several cytoarchitectonic features suggesting cell immaturity, as well as some apparent compensatory plastic responses after ethanol withdrawal. These effects may be due to ethanol impairing neurotrophin-mediated processes during cerebellar development that could lead to alterations in Purkinje cell structure and activity, and thereafter in the psychoneural functions in which the cerebellar cortex is involved.

Lower rhombic lip-derived cells undergo transmedian tangential migration followed by radial migration in the chick embryo brainstem

Migration behaviour and fate of cells originated from the lower rhombic lip (LRL) was examined in the chick embryo hindbrain. LRL-derived cells tangentially migrate along the pial surface of the brainstem and form a transient subpial migratory stream. In the initial stages of migration, LRL-derived cells appose each other or axon-like processes, which is indicative of mode of homophilic chain migration and/or axophilic migration. Some LRL-derived cells relocate rostroventrally towards the pontine region, although the majority of them migrate circumferentially to the ventral medulla oblongata. Depending on the stage of generation, LRL-derived cells undergo transmedian migration; late-generated LRL-derived cells preferentially colonize the contralateral brainstem compared with early generated cells. Thus, latecomer neuron precursors may migrate past their predecessors in the migratory stream. When LRL-derived cells leave the subpial migratory stream, they change their migratory direction to a radial one and relocate inwardly, with a profile that resembles a tangential-to-radial change seen in cerebellar granule cell precursors. After they enter the parenchymal region of the brainstem, they exhibited morphological differentiation, and some differentiate into excitatory neurons. The present results suggest that LRL-derived cells migrate across boundaries such as midline or rhombomere, which may facilitate to build up cellular and functional architectures of the hindbrain.

Dual phases of migration of cerebellar granule cells guided by axonal and dendritic leading processes

During lamination of the cerebellar cortex, granule cells initially migrate tangentially along the external granule layer, and then make a vertical turn and migrate radially to the internal granule layer. We comparatively analyzed the properties of biphasic migration of granule cells in a microexplant culture in which quantitation of morphology and subcellular localization of molecules were readily accomplished. Tangential migration was guided by a leading process that later formed a parallel fiber axon. Translocation of the soma within the axonal process occurred independently of the rapid displacement of the large growth cone. On the other hand, radial migration was guided by a leading process that differentiated into a dendrite after completion of migration. Displacement of the soma and the tiny growth cone were linked so that the radial leading process adopted locomotion and kept a constant length. We propose that the dual phases of granule cell migration are achieved by distinct cellular mechanisms guided by the leading processes forming an axon and a dendrite, respectively.

Neurotrophic activity of pituitary adenylate cyclase-activating polypeptide on rat cerebellar cortex during development

High concentrations of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors are present in the external granule cell layer of the rat cerebellum during postnatal development. In vitro studies have shown that PACAP promotes cell survival and neurite outgrowth on immature cerebellar granule cells in primary culture. In the present study, we have investigated the effect of PACAP on the development of the cerebellar cortex of 8-day-old rats. Incubation of cultured granule cells for 12 or 18 h with PACAP provoked a significant increase in the rate of incorporation of [(3)H]thymidine in cultured granule cells, suggesting that PACAP could stimulate the proliferation of granule cells. After 96 h of treatment, in vivo administration of PACAP provoked a transient increase in the number of granule cells in the molecular layer and in the internal granule cell layer. In contrast, PACAP did not affect the number of Purkinje cells. The augmentation of the number of granule cells evoked by PACAP was significantly inhibited by the PACAP receptor antagonist PACAP(6-38). Administration of PACAP also caused a significant increase in the volume of the cerebellar cortex. The present study provides evidence that PACAP can act in vivo as a trophic factor during rat brain development. Our data indicate that PACAP increases proliferation and/or inhibits programmed cell death of granule cells, as well as stimulating neuronal migration from the external granule cell layer toward the internal granule cell layer.