Abstract
Slit-Robo signaling regulates midline crossing of commissural axons in different systems. In the zebrafish, all retinofugal axons cross at the optic chiasm to innervate the contralateral tectum. Here, the mutant for the Robo2 receptor presents severe axon guidance defects, which were not completely reproduced by a Slit2 ligand null mutant. Since, in the zebrafish, slit3 is also expressed around this area at the stage of axon crossing, we decided to analyze the possibility that it collaborates with Slit2. We found that slit3 expression disruption by sgRNA-Cas9 injection caused similar, albeit slightly milder, defects than those of the slit2 mutant, while the same treatment in the slit2-/- background caused a much more severe defect, comparable to that observed in robo2 mutants. Tracking analysis of in vivo time-lapse experiments indicated differential but complementary functions of these secreted factors in the correction of axon turn errors around the optic chiasm. Interestingly, qPCR analysis showed a mild slit2 expression increase in slit3 deficient embryos, but not the inverse. Our observations support the previously proposed “repulsive channel” model for Slit-Robo action at the optic chiasm, with both Slits acting through Robo2 in different manners, most probably based on their different spatial expression patterns.
Competing Interest Statement
The authors have declared no competing interest.