Microtubule-associated protein 1B: a neuronal binding partner for myelin-associated glycoprotein.

Rachelle Franzen,Richard H Quarles,Suzanne M Dashiell,Jeffrey A Hammer,Sandra L Tanner,Catherine A Rottkamp

doi:10.1083/jcb.200108137

Abstract

Myelin-associated glycoprotein (MAG) is expressed in periaxonal membranes of myelinating glia where it is believed to function in glia–axon interactions by binding to a component of the axolemma. Experiments involving Western blot overlay and coimmunoprecipitation demonstrated that MAG binds to a phosphorylated neuronal isoform of microtubule-associated protein 1B (MAP1B) expressed in dorsal root ganglion neurons (DRGNs) and axolemma-enriched fractions from myelinated axons of brain, but not to the isoform of MAP1B expressed by glial cells. The expression of some MAP1B as a neuronal plasma membrane glycoprotein (Tanner, S.L., R. Franzen, H. Jaffe, and R.H. Quarles. 2000. J. Neurochem. 75:553–562.), further documented here by its immunostaining without cell permeabilization, is consistent with it being a binding partner for MAG on the axonal surface. Binding sites for a MAG-Fc chimera on DRGNs colocalized with MAP1B on neuronal varicosities, and MAG and MAP1B also colocalized in the periaxonal region of myelinated axons. In addition, expression of the phosphorylated isoform of MAP1B was increased significantly when DRGNs were cocultured with MAG-transfected COS cells. The interaction of MAG with MAP1B is relevant to the known role of MAG in affecting the cytoskeletal structure and stability of myelinated axons.

Highlights

The localization of myelin-associated glycoprotein (MAG)* in periaxonal membranes of myelin-forming oligodendrocytes and Schwann cells (SCs) suggests that it functions in glia/axon communication by interacting with a component(s) of the axonal surface membrane
We describe the binding of MAG to proteins of neuronal membrane preparations from the peripheral and central nervous systems, respectively, that would be expected to contain physiological MAG binding partners, i.e., membranes from dorsal root ganglion neurons (DRGNs) and axolemma-enriched fractions (AEFs) from myelinated axons of brain
A previous report from our laboratory (Tanner et al, 2000) and new findings in this paper demonstrate that some microtubule-associated protein 1B (MAP1B) is expressed as a plasma membrane glycoprotein in neurons, supporting the concept that MAP1B is an axonal binding partner for MAG

Summary

JCB Report

Microtubule-associated protein 1B: a neuronal binding partner for myelin-associated glycoprotein. Myelin-associated glycoprotein (MAG) is expressed in periaxonal membranes of myelinating glia where it is believed to function in glia–axon interactions by binding to a component of the axolemma. Experiments involving Western blot overlay and coimmunoprecipitation demonstrated that MAG binds to a phosphorylated neuronal isoform of microtubule-associated protein 1B (MAP1B) expressed in dorsal root ganglion neurons (DRGNs) and axolemma-enriched fractions from myelinated axons of brain, but not to the isoform of MAP1B expressed by glial cells. The expression of some MAP1B as a neuronal plasma membrane glycoprotein Binding sites for a MAG-Fc chimera on DRGNs colocalized with MAP1B on neuronal varicosities, and MAG and MAP1B colocalized in the periaxonal region of myelinated axons. The interaction of MAG with MAP1B is relevant to the known role of MAG in affecting the cytoskeletal structure and stability of myelinated axons

Introduction

Results and discussion

General comments

Materials and methods