The Structural Basis of the Interaction between Myelin-Associated Glyoprotein and Nogo Receptor 2 : Design of Novel Therapeutic Agents for Spinal Cord

Abstract

Thesis (Ph.D.)--University of Rochester. School of Medicine and Dentistry. Dept. of Neurology & Dept. of Biomedical Genetics, 2008.Myelin-associated glycoprotein (MAG) is a sialic acid binding Ig-family lectin that functions in neuronal growth inhibition and stabilization of axon-glia interactions. The ectodomain of MAG is comprised of five Ig-like domains and employs neuronal cell-type specific mechanisms to signal growth inhibition. Here, I show that the first three Ig-like domains of MAG bind with high affinity and in a sialic acid-dependent manner to the Nogo-66 receptor-1 (NgR1) and NgR2. In support of our previous observation that NgR1 is not essential for MAG-elicited neurite outgrowth inhibition, domains Ig4 and Ig5 of MAG, harboring the growth inhibition site, do not associate with NgR1 or NgR2. NgR1 and NgR2 are N- and O-linked sialoglycoproteins comprised of 8.5 leucine-rich repeats followed by a stalk region and a lipid anchor. The stalk of NgR2, but not NgR1, is sufficient for MAG binding and functions as a constitutively active receptor in primary neurons. NgR1 supports binding of MAG, Nogo-66, and OMgp and deletion of disulfide loop Cys309-Cys336 substantially increases the affinity for Nogo-66 and OMgp. A chimeric NgR1 variant (NgROMNI) in which Cys309-Cys336 is deleted and followed by a 13 amino acid MAG binding motif of the NgR2 stalk shows superior binding of OMgp, Nogo-66, and MAG compared to wild-type NgR1 or NgR2. Soluble NgROMNI linked to the Fc portion of human IgG1 (NgROMNI-Fc) binds to membrane bound inhibitors and promotes neurite outgrowth on MAG or myelin substrate in vitro. Thus, NgROMNI-Fc may offer therapeutic opportunities following nervous system injury or disease where myelin inhibits neuronal regeneration