Repulsive signaling from the Drosophila midline requires slit function: Repellent signaling through Robo1 requires the Slit LRR

Abstract

<p>To establish the bilateral symmetry present in the midline of the Drosophila CNS the midline cells must generate a repulsive barrier that divides both halves of the embryo. Here we report the identification and characterization of the midline repellent ligand Slit. Slit is expressed and secreted from the MG where it signals to growth cones pathfinding along the lateral border of the midline. In embryos of severe slit mutants all CNS axons fuse at the midline. slit transgenes expressed at the midline functionally rescue the slit mutant phenotype. Furthermore, ectopic expression of slit transgenes repels CNS axons away from regions of transgene expression. Analysis of point mutations in the slit locus suggest that the LRR motifs found in Slit protein are required for Slit's repulsive function in the midline cells. Additionally, in vivo expression of truncated slit transgenes containing only the LRR was sufficient to partially restore Slit repellent signaling to the midline and generate ectopic signaling phenotypes. slit transgenes lacking the LRR domains have no repellent activity. Embryos transheterozygous for slit and robo1 have pathfinding errors that are not observed in embryos heterozygous for either allele, a classic indication of a genetic interaction between two genes. Additionally, in vitro binding assays demonstrate that Slit binds Robo1 and more specifically that this binding requires the complete Slit LRR domain. These data show that Slit is midline ligand that binds the repellent receptor Robo1 in order to establish the midline barrier formed between opposite halves of the Drosophila embryo.</p>