The myelin membrane influences the organization of molecules on the axonal surface

Abstract

The paranodal axoglial junction, a highly unique and specialized vertebrate structure, is the most likely domain at which axon‐glial communication takes place. The molecular components of this structure are largely unknown. The first identified protein that was localized specifically to the axoglial junction was caspr/paranodin, on the axonal side. The second was neurofascin 155 (NF155), a member of the immunoglobulin superfamily of cell adhesion molecules, on the glial cell side. In the mature myelinating glial cell, NF155 clusters in apposition to axonal caspr at the paranodal junction. These proteins are believed to be important in the establishment and stabilization of the axo‐glial contact and in the restriction of sodium channels to the nodal region. We have studied the spatial and temporal organization of the myelin membrane and the way myelin influences the organization of the axon, in particular of membrane proteins at the nodal and paranodal level. We observed that there is a focal increase of caspr along axonal segments that are in contact with oligodendrocyte membrane expansions. This up‐regulation or redistribution of caspr on the axonal side is mirrored by an increased of NF155 on the oligodendrocyte side. Therefore, caspr and NF155 codistribute from the time of initial contact, until they reach their final distribution at the paranodal junction. This implicates NF155 and caspr in axoglial recognition and early communication, as well as in the establishment of membrane compartments or domains, where molecules are restricted from freely diffusing in the lateral plane of the axonal membrane.