Retinal ganglion cell axons exit the eye, enter the optic stalk, cross the ventral midline at the optic chiasm, and terminate in the optic tectum of the zebrafish. While in the optic stalk, they grow immediately adjacent to cells expressing the powerful retinal axon repellent slit2. The chemokine stromal cell-derived factor-1 (SDF1) is expressed within the optic stalk and its receptor CXCR4 is expressed in retinal ganglion cells. SDF1 makes cultured retinal axons less responsive to slit2. Here, we show that reducing SDF1 signaling in vivo rescues retinal axon pathfinding errors in zebrafish mutants that have a partial functional loss of the slit receptor robo2. In contrast, reducing SDF1 signaling in animals that completely lack the robo2 receptor does not rescue retinal guidance errors. These results demonstrate that endogenous levels of SDF1 antagonize the repellent effects of slit/robo signaling in vivo and that this antagonism is important during axonal pathfinding.

During formation of the optic projection in astray/robo2 mutant zebrafish, optic axons exhibit rostro-caudal pathfinding errors, ectopic midline crossing and increased terminal arbor size. Here we show that these errors persist into adulthood, even when robo2 function is conditionally reduced only during initial formation of the optic projection. Adult errors include massive ectopic optic tracts in the telencephalon. During optic nerve regeneration in astray/robo2 animals, these tracts are not re-populated and ectopic midline crossing is reduced compared to unlesioned mutants. This is despite a comparable macrophage/microglial response and upregulation of contactin1a in oligodendrocytes of entopic and ectopic tracts. However, other errors, such as expanded termination areas and ectopic growth into the tectum, were frequently re-committed by regenerating optic axons. Retinal ganglion cells with regenerating axons re-express robo2 and expression of slit ligands is maintained in some areas of the adult optic pathway. However, slit expression is reduced rostral and caudal to the chiasm, compared to development and ubiquitous overexpression of Slit2 did not elicit major pathfinding phenotypes. This shows that (1) there is not an efficient correction mechanism for large-scale pathfinding errors of optic axons during development; (2) degenerating tracts do not provide a strong guidance cue for regenerating optic axons in the adult CNS, unlike the PNS; and (3) robo2 is less important for pathfinding of optic axons during regeneration than during development.

At the optic chiasm, axons from either eye meet and decide whether to cross contralaterally or turn back ipsilaterally. Here, the guidance ligand Slit and its receptor Robo control not whether axons cross (as in other midline decisions), but where the chiasm forms. Whether axons cross is instead controlled by the transcription factor Zic2 and the guidance receptor EphB1, as shown by two papers in the current issues of Neuron and Cell (Herrera et al. and Williams et al.). Surprisingly, this mechanism is conserved evolutionarily from frogs to mammals.