title: Pioneer midbrain longitudinal axons navigate using a balance of Netrin attraction and Slit repulsion.
abstract: BACKGROUND
  Longitudinal axons grow parallel to the embryonic midline to connect distant regions of the central nervous system. Previous studies suggested that repulsive midline signals guide pioneer longitudinal axons by blocking their entry into the floor plate; however, the role of midline attractants, and whether attractant signals may cooperate with repulsive signals, remains unclear. In this study we investigated the navigation of a set of pioneer longitudinal axons, the medial longitudinal fasciculus, in mouse embryos mutant for the Netrin/Deleted in Colorectal Cancer (DCC) attractants, and for Slit repellents, as well as the responses of explanted longitudinal axons in vitro.
  
  
  RESULTS
  In mutants for Netrin1 chemoattractant or DCC receptor signaling, longitudinal axons shifted away from the ventral midline, suggesting that Netrin1/DCC signals act attractively to pull axons ventrally. Analysis of mutants in the three Slit genes, including Slit1/2/3 triple mutants, suggest that concurrent repulsive Slit/Robo signals push pioneer axons away from the ventral midline. Combinations of mutations between the Netrin and Slit guidance systems provided genetic evidence that the attractive and repulsive signals balance against each other. This balance is demonstrated in vitro using explant culture, finding that the cues can act directly on longitudinal axons. The explants also reveal an unexpected synergy of Netrin1 and Slit2 that promotes outgrowth.
  
  
  CONCLUSIONS
  These results support a mechanism in which longitudinal trajectories are positioned by a push-pull balance between opposing Netrin and Slit signals. Our evidence suggests that longitudinal axons respond directly and simultaneously to both attractants and repellents, and that the combined signals constrain axons to grow longitudinally.
authors: Minkyung
authors: M
authors: Kim
authors: W Todd
authors: WT
authors: Farmer
authors: Brielle
authors: B
authors: Bjorke
authors: Samuel A
authors: SA
authors: McMahon
authors: Pierre J
authors: PJ
authors: Fabre
authors: Frédéric
authors: F
authors: Charron
authors: Department of Biology, University of Nevada, 1664 N Virginia St, Reno, NV 89557, USA. gmastick@unr.edu.
authors: Grant S
authors: GS
authors: Mastick
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology*; D002633:Chemotaxis; D036341:Intercellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D008636:Mesencephalon / Q000196:embryology* / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009414:Nerve Growth Factors / Q000235:genetics / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D000075388:Netrin-1; D009474:Neurons / Q000166:cytology / Q000502:physiology; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000378:metabolism*; D000098552:Slit Homolog 2 Protein
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D009414:Nerve Growth Factors; D009419:Nerve Tissue Proteins; C120010:Slit1 protein, mouse; C432251:Slit3 protein, mouse; D025521:Tumor Suppressor Proteins; D000075388:Netrin-1; D000098552:Slit Homolog 2 Protein