title: RabGDI controls axonal midline crossing by regulating Robo1 surface expression.
abstract: BACKGROUND
  Axons navigate to their future synaptic targets with the help of choice points, intermediate targets that express axon guidance cues. Once they reach a choice point, axons need to switch their response from attraction to repulsion in order to move on with the next stage of their journey. The mechanisms underlying the change in axonal responsiveness are poorly understood. Commissural axons become sensitive to the repulsive activity of Slits when they cross the ventral midline of the CNS. Responsiveness to Slits depends on surface expression of Robo receptors. In Drosophila, Commissureless (Comm) plays a crucial regulatory role in midline crossing by keeping Robo levels low on precommissural axons. Interestingly, to date no vertebrate homolog of comm has been identified. Robo3/Rig1 has been shown to control Slit sensitivity before the midline, but without affecting Robo1 surface expression.
  
  
  RESULTS
  We had identified RabGDI, a gene linked to human mental retardation and an essential component of the vesicle fusion machinery, in a screen for differentially expressed floor-plate genes. Downregulation of RabGDI by in ovo RNAi caused commissural axons to stall in the floor plate, phenocopying the effect observed after downregulation of Robo1. Conversely, premature expression of RabGDI prevented commissural axons from entering the floor plate. Furthermore, RabGDI triggered Robo1 surface expression in cultured commissural neurons. Taken together, our results identify RabGDI as a component of the switching mechanism that is required for commissural axons to change their response from attraction to repulsion at the intermediate target.
  
  
  CONCLUSIONS
  RabGDI takes over the functional role of fly Comm by regulating the surface expression of Robo1 on commissural axons in vertebrates. This in turn allows commissural axons to switch from attraction to repulsion at the midline of the spinal cord.
authors.affiliation: Institute of Molecular Life Sciences, University of Zurich, Winterthurerstrasse 190, Zurich, CH 8057, Switzerland.
authors.forename: Melanie
authors.initials: M
authors.lastname: Philipp
authors.forename: Vera
authors.initials: V
authors.lastname: Niederkofler
authors.forename: Marc
authors.initials: M
authors.lastname: Debrunner
authors.forename: Tobias
authors.initials: T
authors.lastname: Alther
authors.forename: Beat
authors.initials: B
authors.lastname: Kunz
authors.forename: Esther T
authors.initials: ET
authors.lastname: Stoeckli
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology*; D002461:Cell Line, Transformed; D002465:Cell Movement / Q000235:genetics; D002642:Chick Embryo; D002522:Chlorocebus aethiops; D007839:Functional Laterality / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D020730:Guanine Nucleotide Dissociation Inhibitors / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D009474:Neurons / Q000166:cytology*; D034622:RNA Interference / Q000502:physiology; D011971:Receptors, Immunologic / Q000378:metabolism*; D013116:Spinal Cord / Q000166:cytology / Q000196:embryology / Q000254:growth & development; D014162:Transfection; D014186:Transportation; D000097905:Roundabout Proteins
chemical_list: D020730:Guanine Nucleotide Dissociation Inhibitors; D009419:Nerve Tissue Proteins; D011971:Receptors, Immunologic; D000097905:Roundabout Proteins; C086796:GDP dissociation inhibitor 1