Squeezing axons out of the gray matter: a role for slit and semaphorin proteins from midline and ventral spinal cord.
Commissural axons cross the nervous system midline and then turn to grow alongside it, neither recrossing nor projecting back into ventral regions. In Drosophila, the midline repellent Slit prevents recrossing: axons cross once because they are initially unresponsive to Slit, becoming responsive only upon crossing. We show that commissural axons in mammals similarly acquire responsiveness to a midline repellent activity upon crossing. Remarkably, they also become responsive to a repellent activity from ventral spinal cord, helping explain why they never reenter that region. Several Slit and Semaphorin proteins, expressed in midline and/or ventral tissues, mimic these repellent activities, and midline guidance defects are observed in mice lacking neuropilin-2, a Semaphorin receptor. Thus, Slit and Semaphorin repellents from midline and nonmidline tissues may help prevent crossing axons from reentering gray matter, squeezing them into surrounding fiber tracts.
Zou
Y
Y
Department of Anatomy, Howard Hughes Medical Institute, University of California, San Francisco 94143, USA.
Stoeckli
E
E
Chen
H
H
Tessier-Lavigne
M
M
D000818:Animals; D001369:Axons; D002450:Cell Communication; D029721:Drosophila Proteins; D066298:In Vitro Techniques; D036341:Intercellular Signaling Peptides and Proteins; D008969:Molecular Sequence Data; D009024:Morphogenesis; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D009434:Neural Pathways / Q000166:cytology / Q000196:embryology*; D039942:Neuropilin-1; D051381:Rats; D013116:Spinal Cord / Q000166:cytology / Q000196:embryology*; D000098552:Slit Homolog 2 Protein
D029721:Drosophila Proteins; D036341:Intercellular Signaling Peptides and Proteins; D009419:Nerve Tissue Proteins; C066819:sli protein, Drosophila; D039942:Neuropilin-1; D000098552:Slit Homolog 2 Protein