Disclosed herein are methods and compositions for increasing and decreasing the permeability of the blood brain barrier for the treatment of diseases and conditions and to facilitate the delivery of agents to the brain, as well as methods and compositions for promoting re-myelination and preventing de-myelination. Compositions include RGMa, soluble RGMa, and functional fragments and variants thereof, RGMc, soluble RGMc, and functional fragments and variants thereof, and Neogenin peptides including 4Ig.