Key role of Rho GTPases in motor disorders associated with neurodevelopmental pathologies.

Growing evidence suggests that Rho GTPases and molecules involved in their signaling pathways play a major role in the development of the central nervous system (CNS). Whole exome sequencing (WES) and de novo examination of mutations, including SNP (Single Nucleotide Polymorphism) in genes coding for the molecules of their signaling cascade, has allowed the recent discovery of dominant autosomic mutations and duplication or deletion of candidates in the field of neurodevelopmental diseases (NDD). Epidemiological studies show that the co-occurrence of several of these neurological pathologies may indeed be the rule.

A genome-wide search for new imprinted genes in the human placenta identifies DSCAM as the first imprinted gene on chromosome 21.

We identified, through a genome-wide search for new imprinted genes in the human placenta, DSCAM (Down Syndrome Cellular Adhesion Molecule) as a paternally expressed imprinted gene. Our work revealed the presence of a Differentially Methylated Region (DMR), located within intron 1 that might regulate the imprinting in the region. This DMR showed a maternal allele methylation, compatible with its paternal expression.

Trio GEF mediates RhoA activation downstream of Slit2 and coordinates telencephalic wiring.

Trio, a member of the Dbl family of guanine nucleotide exchange factors, activates Rac1 downstream of netrin 1/DCC signalling in axon outgrowth and guidance. Although it has been proposed that Trio also activates RhoA, the putative upstream factors remain unknown. Here, we show that Slit2 induces Trio-dependent RhoA activation, revealing a crosstalk between Slit and Trio/RhoA signalling.

KCC2 Gates Activity-Driven AMPA Receptor Traffic through Cofilin Phosphorylation.

Expression of the neuronal K/Cl transporter KCC2 is tightly regulated throughout development and by both normal and pathological neuronal activity. Changes in KCC2 expression have often been associated with altered chloride homeostasis and GABA signaling. However, recent evidence supports a role of KCC2 in the development and function of glutamatergic synapses through mechanisms that remain poorly understood.

Mechanisms controlling neuromuscular junction stability.

The neuromuscular junction (NMJ) is the synaptic connection between motor neurons and muscle fibers. It is involved in crucial processes such as body movements and breathing. Its proper development requires the guidance of motor axons toward their specific targets, the development of multi-innervated myofibers, and a selective synapse stabilization.

Hyperactivation of Alk induces neonatal lethality in knock-in AlkF1178L mice.

The ALK (Anaplastic Lymphoma Kinase) gene encodes a tyrosine kinase receptor preferentially expressed in the central and peripheral nervous systems. A syndromic presentation associating congenital neuroblastoma with severe encephalopathy and an abnormal shape of the brainstem has been described in patients harbouring de novo germline F1174V and F1245V ALK mutations. Here, we investigated the phenotype of knock-in (KI) mice bearing the AlkF1178L mutation (F1174L in human).

CLIPR-59: a protein essential for neuromuscular junction stability during mouse late embryonic development.

CLIPR-59 is a new member of the cytoplasmic linker proteins (CLIP) family mainly localized to the trans-Golgi network. We show here that Clipr-59 expression in mice is restricted to specific pools of neurons, in particular motoneurons (MNs), and progressively increases from embryonic day 12.5 (E12.

Origin and plasticity of the subdivisions of the inferior olivary complex.

The precerebellar nuclei (PCN) originate from the rhombic lip, a germinal neuroepithelium adjacent to the roof plate of the fourth ventricle. We first report here that, in chicken, the Brn3a-expressing postmitotic medullary cells that produce the inferior olive (ION, the source of cerebellar climbing fibres) originate from a dorso-ventral domain roughly coinciding with the hindbrain vestibular column. Whereas Foxd3 expression labels the whole mature ION but is only detected in a subpopulation of ION neuroblasts initiating their migration, we report that Brn3a allows the visualization of the whole population of ION neurons from the very beginning of their migration.

The vesicular SNARE Synaptobrevin is required for Semaphorin 3A axonal repulsion.

Attractive and repulsive molecules such as Semaphorins (Sema) trigger rapid responses that control the navigation of axonal growth cones. The role of vesicular traffic in axonal guidance is still largely unknown. The exocytic vesicular soluble N-ethylmaleimide sensitive fusion protein attachment protein receptor (SNARE) Synaptobrevin 2 (Syb2) is known for mediating neurotransmitter release in mature neurons, but its potential role in axonal guidance remains elusive.

Differential roles of Netrin-1 and its receptor DCC in inferior olivary neuron migration.

Netrin-1 was previously shown to be required for the tangential migration and survival of neurons that will form the inferior olivary nucleus (ION). Surprisingly, the compared analysis of mutant mice lacking either Netrin-1 or its major receptor DCC reveals striking phenotypic differences besides common features. Although ectopic stops of ION cell bodies occur in the same positions along the migratory stream in both mutants, the ION neurons' number is not affected by the lack of DCC whereas it is reduced in Netrin-1 mutant mice.

Tubulin tyrosination is required for the proper organization and pathfinding of the growth cone.

Background: During development, neuronal growth cones integrate diffusible and contact guidance cues that are conveyed to both actin and microtubule (MT) cytoskeletons and ensure axon outgrowth and pathfinding. Although several post-translational modifications of tubulin have been identified and despite their strong conservation among species, their physiological roles during development, especially in the nervous sytem, are still poorly understood.

Methodology/findings: Here, we have dissected the role of a post-translational modification of the last amino acid of the alpha-tubulin on axonal growth by analyzing the phenotype of precerebellar neurons in Tubulin tyrosin ligase knock-out mice (TTL(-/-)) through in vivo, ex vivo and in vitro analyses.

Trio controls the mature organization of neuronal clusters in the hindbrain.

During the embryonic development of the hindbrain, movements of neuronal clusters allow the formation of mature "pools", in particular for inferior olivary (ION) and facial motor (fMN) nuclei. The cellular mechanisms of neuron clustering remain uncharacterized. We report that the absence of the Rho-guanine exchange factor Trio, which can activate both RhoG and Rac1 in vivo, prevents the proper formation of ION and fMN subnuclei.

Development of precerebellar nuclei: instructive factors and intracellular mediators in neuronal migration, survival and axon pathfinding.

The precerebellar system provides an interesting model to study tangential migrations. All precerebellar neurons (PCN) are generated in the most alar part of the hindbrain in a region called rhombic lip. PCN first emit a leading process and then translocate their nuclei inside it, a mechanism called nucleokinesis.

Specification of the meso-isthmo-cerebellar region: the Otx2/Gbx2 boundary.

The midbrain/hindbrain (MH) territory containing the mesencephalic and isthmocerebellar primordial is characterized by the expression of several families of regulatory genes including transcription factors (Otx, Gbx, En, and Pax) and signaling molecules (Fgf and Wnt). At earlier stages of avian neural tube, those genes present a dynamic expression pattern and only at HH18-20 onwards, when the mesencephalic/metencephalic constriction is coincident with the Otx2/Gbx2 boundary, their expression domains become more defined. This review summarizes experimental data concerning the genetic mechanisms involved in the specification of the midbrain/hindbrain territory emphasizing the chick/quail chimeric experiments leading to the discovery of a secondary isthmic organizer.

Phosphorylation of DCC by Fyn mediates Netrin-1 signaling in growth cone guidance.

Netrin-1 acts as a chemoattractant molecule to guide commissural neurons (CN) toward the floor plate by interacting with the receptor deleted in colorectal cancer (DCC). The molecular mechanisms underlying Netrin-1-DCC signaling are still poorly characterized. Here, we show that DCC is phosphorylated in vivo on tyrosine residues in response to Netrin-1 stimulation of CN and that the Src family kinase inhibitors PP2 and SU6656 block both Netrin-1-dependent phosphorylation of DCC and axon outgrowth.

Distinct roles of Rac1/Cdc42 and Rho/Rock for axon outgrowth and nucleokinesis of precerebellar neurons toward netrin 1.

During embryonic development, tangentially migrating precerebellar neurons emit a leading process and then translocate their nuclei inside it (nucleokinesis). Netrin 1 (also known as netrin-1) acts as a chemoattractant factor for neurophilic migration of precerebellar neurons (PCN) both in vivo and in vitro. In the present work, we analyzed Rho GTPases that could direct axon outgrowth and/or nuclear migration.

Nr-CAM and TAG-1 are expressed in distinct populations of developing precerebellar and cerebellar neurons.

Nr-CAM and TAG-1 interact at the floor-plate during the formation of spinal cord commissural projections [Stoeckli, E.T., Landmesser, L.

Directional guidance of oligodendroglial migration by class 3 semaphorins and netrin-1.

Oligodendrocytes, the myelin-forming cells of the CNS, are generated from multiple foci distributed along the developing neural tube. Little is known about the endogenous guidance cues controlling the migration of oligodendrocyte precursor cells (OPCs) from their site of emergence toward their final destination, mainly the future white matter tracts. During embryonic development, the optic nerve is populated by OPCs originating in the diencephalon that migrate from the chiasm toward the retina.

Slit antagonizes netrin-1 attractive effects during the migration of inferior olivary neurons.

Inferior olivary neurons (ION) migrate circumferentially around the caudal rhombencephalon starting from the alar plate to locate ventrally close to the floor-plate, ipsilaterally to their site of proliferation. The floor-plate constitutes a source of diffusible factors. Among them, netrin-1 is implied in the survival and attraction of migrating ION in vivo and in vitro.

Netrin-1 acts as a survival factor via its receptors UNC5H and DCC.

The membrane receptors DCC and UNC5H have been shown to be crucial for axon guidance and neuronal migration by acting as receptors for netrin-1. DCC has also been proposed as a dependence receptor inducing apoptosis in cells that are beyond netrin-1 availability. Here we show that the netrin-1 receptors UNC5H (UNC5H1, UNC5H2, UNC5H3) also act as dependence receptors.

[Formation of the boundary between the midbrain and the hindbrain: involvement of Otx2 and Gbx2 genes].

We have studied the neuromeric organisation of the mesencephalic-metencephalic (mes-met) territory of the avian neural tube using chick/quail transplantation experiments and analysing the expression of various regulatory genes in chimeric and normal embryos. Homotopic grafts demonstrate the presence of an interneuromeric boundary separating the mesencephalic and cerebellar territories (the mes-met or midbrain/hindbrain boundary). This boundary is characterised from HH10 onwards by the confrontation of the Otx2-Wnt1 and Gbx2-Fgf8 expressing domains, while En2 and Pax2 genes are expressed at both sides of the mes-met boundary.

Floor plate and netrin-1 are involved in the migration and survival of inferior olivary neurons.

During their circumferential migration, the nuclei of inferior olivary neurons translocate within their axons until they reach the floor plate where they stop, although their axons have already crossed the midline to project to the contralateral cerebellum. Signals released by the floor plate, including netrin-1, have been implicated in promoting axonal growth and chemoattraction during axonal pathfinding in different midline crossing systems. In the present study, we report experiments that strongly suggest that the floor plate could also be involved in the migration of inferior olivary neurons.

The embryonic cerebellum contains topographic cues that guide developing inferior olivary axons.

The formation of the olivocerebellar projection is supposed to be regulated by positional information shared between pre- and postsynaptic neurons. However, experimental evidence to support this hypothesis is missing. In the chick, caudal neurons in the inferior olive project to the anterior cerebellum and rostral ones to the posterior cerebellum.