Figure 9.

Inhibiting LRPs/F-spondin binding enables growth of commissural axons into the floor plate cells. ApoER2^ecto (A–C) or F-spondin EGFP-tagged isoforms (D–L) were electroporated into the ventral spinal cord at stages 12–14. Cross-sections of stage 22–24 embryos were stained with anti-EGFP (A, D, G, and J) or antineurofilament mAb 3A10 (B, E, H, and K). In the control experiment, expression of EGFP (J–L), commissural axons cross the midline as a tight bundle (arrowhead) under the floor plate cells whereas expression of ApoER2^ecto (A–C) and the mutated F-spondin's isoforms (D–L) resulted in a dorsal erroneous neurites projection into the floor plate cells (B, E, and H, arrows). (M) Quantification of the extent of pathfinding errors at the midline. For each protein ∼100 sections were inspected. A cross-section with axons extending dorsally at the floor plate was scored as an error. The crossed arrows indicate point mutations in the plasmin cleavage sites. Bar, 50 μm.