P133. The role of Nogo on the guidance of commissural axons in the mouse spinal cord

Abstract

Nogo is a member of reticulon superfamily proteins that consists of 3 isoforms, Nogo-A, -B and -C. It inhibits axon regeneration in the adult central nervous system. Previous studies in our laboratory showed Nogo is an inhibitory molecule regulating axon divergence in the mouse optic pathway. Here we found that this inhibitory molecule was also expressed in the developing spinal cord and involved in commissural axon guidance. Using Nogo specific antibodies, both Nogo-A and Nogo-B were localized on neurons and radial glial cells. In the floor plate, only Nogo-B was localized on the radial glia processes when commissural axons are crossing the midline. Nogo-66 receptor (NgR) was expressed weakly by commissural axons initially but was upregulated after crossing. After blocking NgR activity with NEP1-40, midline crossing was dramatically reduced in open-book preparations of the spinal cord, both at E11 and E12. Growth cones stalled in the midline region were more complex in morphology and larger in size when compared with those in controls treated without the blocking peptide. In co-culture assays, E13 floor plate inhibited extension of post-commissural axons, and this inhibition was abolished by NEP1-40. Outgrowth of pre-commissural axons was enhanced in the presence of floor plate. Western-blot assays of conditioned medium collected from floor plate cultures showed that a 37-kDa form of Nogo that was secreted by the floor plate. These results suggest that Nogo is involved in repelling commissural axons out of the floor plate through NgR, and that this repulsion may be mediated by contact inhibition through membrane bound Nogo and long range influence through diffusible form of the protein. This project is supported partially by a General Research Fund (Ref. no. CUHK461709) from the Research Grant Council of the Hong Kong Administrative Region.