No abstract

Netrin-1 induces repulsive axon guidance by binding to the mammalian Unc5 receptor family (Unc5A-Unc5D). Mouse genetic analysis of selected members of the Unc5 family, however, revealed essential functions independent of Netrin-1, suggesting the presence of other ligands. Unc5B was recently shown to bind fibronectin and leucine-rich transmembrane protein-3 (FLRT3), although the relevance of this interaction for nervous system development remained unclear. Here, we show that the related Unc5D receptor binds specifically to another FLRT protein, FLRT2. During development, FLRT2/3 ectodomains (ECDs) are shed from neurons and act as repulsive guidance molecules for axons and somata of Unc5-positive neurons. In the developing mammalian neocortex, Unc5D is expressed by neurons in the subventricular zone (SVZ), which display delayed migration to the FLRT2-expressing cortical plate (CP). Deletion of either FLRT2 or Unc5D causes a subset of SVZ-derived neurons to prematurely migrate towards the CP, whereas overexpression of Unc5D has opposite effects. Hence, the shed FLRT2 and FLRT3 ECDs represent a novel family of chemorepellents for Unc5-positive neurons and FLRT2/ Unc5D signalling modulates cortical neuron migration.

Neuronal network formation in the developing nervous system is dependent on the accurate navigation of nerve cell axons and dendrites, which is controlled by attractive and repulsive guidance cues. Ephrins and their cognate Eph receptors mediate many repulsive axonal guidance decisions by intercellular interactions resulting in growth cone collapse and axon retraction of the Eph-presenting neuron. We show that the Rac-specific GTPase-activating protein alpha2-chimaerin binds activated EphA4 and mediates EphA4-triggered axonal growth cone collapse. alpha-Chimaerin mutant mice display a phenotype similar to that of EphA4 mutant mice, including aberrant midline axon guidance and defective spinal cord central pattern generator activity. Our results reveal an alpha-chimaerin-dependent signaling pathway downstream of EphA4, which is essential for axon guidance decisions and neuronal circuit formation in vivo.

The members of the glial cell line-derived neurotrophic factor (GDNF) family of neurotrophic factors (GDNF, neurturin, persephin, and artemin) are able to promote in vivo and in vitro survival of different neuronal populations, including spinal cord motoneurons. These factors signal via multicomponent receptors that consist of the Ret receptor tyrosine kinase plus a member of the GDNF family receptor ␣ (GRF␣) family of glycosylphosphatidylinositol-linked coreceptors. Activation of the receptor induces Ret phosphorylation that leads the survival-promoting effects. Ret phosphorylation causes the activation of several intracellular pathways, but the biological effects caused by the activation of each of these pathways are still unknown. In the present work, we describe the ability of the GDNF family members to promote chicken motoneuron survival in culture. We show the presence of Ret and GFR␣-1, GFR␣-2, and GFR␣-4 in chicken motoneurons using in situ hybridization and reverse transcription-PCR techniques. By Western blot analysis and kinase assays, we demonstrate the ability of these factors to induce the phosphatidylinositol 3 kinase (PI 3-kinase) and the extracellular regulated kinase (ERK)-mitogen-activated protein (MAP) kinase pathways activation. To characterize the involvement of these pathways in the survival effect, we used the PI 3-kinase inhibitor LY 294002 and the MAP kinase and ERK kinase (MEK) inhibitor PD 98059. We demonstrate that LY 294002, but not PD 98059, prevents GDNF-, neurturin-, and persephin-induced motoneuron survival, suggesting that PI 3-kinase intracellular pathway is responsible in mediating the neurotrophic effect. Key words: GDNF; persephin; neurturin; artemin; motoneuron; neurotrophic factor; GFR␣ receptors; chicken; intracellular signaling pathwayGlial cell line-derived neurotrophic factor (GDN F), neurturin (NTN), persephin (PSP), and artemin (ART) are the members of a new family of neurotrophic factors distantly related to the members of the TGF␤ family (Lin et al