Neurofascin Is a Glial Receptor for the Paranodin/Caspr-Contactin Axonal Complex at the Axoglial Junction

Abstract

In myelinated fibers of the vertebrate nervous system, glial-ensheathing cells interact with axons at specialized adhesive junctions, the paranodal septate-like junctions [1]. The axonal proteins paranodin/Caspr and contactin form a cis complex in the axolemma at the axoglial adhesion zone, and both are required to stabilize the junction [2, 3]. There has been intense speculation that an oligodendroglial isoform of the cell adhesion molecule neurofascin, NF155, expressed at the paranodal loop [4, 5] might be the glial receptor for the paranodin/Caspr-contactin complex, particularly since paranodin/Caspr and NF155 colocalize to ectopic sites in the CNS of the dysmyelinated mouse Shiverer mutant [5]. We report that the extracellular domain of NF155 binds specifically to transfected cells expressing the paranodin/Caspr-contactin complex at the cell surface. This region of NF155 also binds the paranodin/Caspr-contactin complex from brain lysates in vitro. In support of the functional significance of this interaction, NF155 antibodies and the extracellular domain of NF155 inhibit myelination in myelinating cocultures, presumably by blocking the adhesive relationship between the axon and glial cell. These results demonstrate that the paranodin/Caspr-contactin complex interacts biochemically with NF155 and that this interaction is likely to be biologically relevant at the axoglial junction.