Moreover, the FLK1-binding VEGF120 isoform did not promote axon growth or growth cone turning in vitro. These findings suggest that NRP1 controls the behavior

of developing RGC axons independently of its vascular coreceptor FLK1, or indeed FLT1, which also is not expressed by developing RGCs. Future studies might therefore examine if NRP1 in RGC axons signals through its cytoplasmic tail or recruits a coreceptor that is not a classical VEGF receptor. VEGF164 has been hypothesized to regulate axon guidance based on its ability to compete with SEMA3A for NRP1 binding (Carmeliet, 2003). However, we could not identify an selleck screening library essential role for SEMA signaling through NRP1 in optic chiasm development in mice. Accordingly, neither the genetic ablation of SEMA3A, nor the loss of SEMA signaling through NRP1 alone or both neuropilins together, perturbed optic chiasm development. These findings were surprising, because the NRP1 ligand

SEMA3D provides repulsive signals that channel RGC axons into the contralateral optic tract in zebrafish (Seth et al., buy AZD2281 2006). A possible explanation for the class 3 SEMA requirement in fish, but not mammals, is that fish have an exclusive contralateral projection. It will therefore be interesting to investigate whether VEGF-A signaling at the chiasm midline is conserved in all vertebrates, independently of SEMAs, or if there is a species-dependent specialization with respect to the choice of NRP1 ligand. Interestingly, even Drosophila, a species without a circulatory system, has a VEGF-A Axenfeld syndrome homolog that promotes cell migration ( Traver and Zon, 2002). This raises the possibility that VEGF-A plays evolutionary conserved roles in the nervous system that predates its function in blood vessels. Previous in vitro experiments raised the possibility that a growth-promoting factor for commissural axons is present at the chiasm

midline (Tian et al., 2008). However, the molecular identity of this factor has never been established. The only molecule found previously to promote contralateral RGC axon growth is the cell adhesion molecule NrCAM. However, NrCAM is not the elusive midline cue that promotes commissural axon crossing at the optic chiasm, because it acts as a receptor within RGC axons rather than as a guidance signal at the chiasm midline (Williams et al., 2006). In the vertebrate spinal cord, commissural axons are attracted to the midline by the combined action of the chemoattractants netrin 1 and SHH (Serafini et al., 1996 and Charron et al., 2003). However, neither of these molecules is expressed at the chiasm midline or promotes contralateral RGC axon extension (Deiner and Sretavan, 1999, Marcus et al., 1999, Trousse et al., 2001 and Sánchez-Camacho and Bovolenta, 2008).