5 μg/ml function-blocking goat anti-rat NRP1 antibody or control IgG was added. The angle turned by the growth cone was calculated using Image J. Statistical comparisons were made using a Mann-Whitney U test. We thank Drs. A.L. Kolodkin, D.D. Ginty, C. Gu, H. Fujisawa, J. Rossant, G.H. Fong, and M. Taniguchi for mouse strains; the staff of the Biological

Resources Unit at the UCL Institute of Ophthalmology for help with mouse husbandry; the Institute of Medical Sciences Microscopy and Imaging Facility for help with confocal microscopy; and Kathryn Davidson, Heather Walker, and Andrew Peace for technical assistance. This research was funded by a Wellcome Trust Project Grant to L.E. and C.R. (reference 085476) and a Central VX-809 manufacturer Research Fund grant from the University of London to C.R. (reference AR/CRF/B). “
“During developmental wiring of the nervous system, axons respond to attractive and repulsive guidance cues to navigate to their targets. Surprisingly, only a small number of guidance cues have been identified so far, suggesting that additional chemoattractants and repellents remain to be discovered. A well-known model system to study axon guidance is the spinal cord ventral midline. During development, commissural neurons, located in the dorsal spinal cord, send axons that project toward and subsequently across the floor plate, a find more specialized structure at the ventral midline, which acts as an intermediate target and influences commissural axons

by expressing attractive and repulsive cues (Dickson and

Zou, 2010). The first midline guidance cue identified, Netrin-1, has two distinct activities on precrossing commissural axons: it stimulates growth and attracts these axons toward the floor plate (reviewed in Charron and Tessier-Lavigne, 2005). Precrossing commissural axons are also guided by Sonic hedgehog (Shh), which chemoattracts commissural axons without stimulating their growth (Charron et al., 2003). Although Shh and Netrin-1 are required for normal guidance of commissural axons, intriguingly, when dorsal spinal cord explants are exposed to Netrin-1-deficient floor plates in the presence of Shh signaling inhibitors, some commissural axons are Bay 11-7085 still attracted (Charron et al., 2003). This suggests that the floor plate secretes other chemoattractants than Netrin-1 and Shh. However, the molecular nature of these floor plate-derived attractant guidance cues remains unknown. Increasing evidence indicates that vascular endothelial growth factor A (VEGF-A, termed VEGF from hereon), a prototypic angiogenic factor, plays a key role in the nervous system (Ruiz de Almodovar et al., 2009). For instance, VEGF promotes proliferation, migration, differentiation and survival of neuroblasts (Jin et al., 2002, Wittko et al., 2009 and Zhang et al., 2003), and induces axonal outgrowth of various neurons (Ruiz de Almodovar et al., 2009). By activating its signaling receptor Flk1, VEGF chemoattracts cerebellar granule cells (Ruiz de Almodovar et al.