Myelin-associated Glycoprotein Interacts with Ganglioside GT1b: A MECHANISM FOR NEURITE OUTGROWTH INHIBITION

Mary Vinson,Paul J.L.M Strijbos,Alison Rowles,Laura Facci,Stephen E Moore,David L Simmons,Frank S Walsh

doi:10.1074/jbc.m100345200

Mary Vinson, Paul J.L.M Strijbos + Show 5 more

Open Access

https://doi.org/10.1074/jbc.m100345200

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Abstract

Myelin-associated glycoprotein (MAG) is expressed on myelinating glia and inhibits neurite outgrowth from post-natal neurons. MAG has a sialic acid binding site in its N-terminal domain and binds to specific sialylated glycans and gangliosides present on the surface of neurons, but the significance of these interactions in the effect of MAG on neurite outgrowth is unclear. Here we present evidence to suggest that recognition of sialylated glycans is essential for inhibition of neurite outgrowth by MAG. Arginine 118 on MAG is known to make a key contact with sialic acid. We show that mutation of this residue reduces the potency of MAG inhibitory activity but that residual activity is also a result of carbohydrate recognition. We then go on to investigate gangliosides GT1b and GD1a as candidate MAG receptors. We show that MAG specifically binds both gangliosides and that both are expressed on the surface of MAG-responsive neurons. Furthermore, antibody cross-linking of cell surface GT1b, but not GD1a, mimics the effect of MAG, in that neurite outgrowth is inhibited through activation of Rho kinase. These data strongly suggest that interaction with GT1b on the neuronal cell surface is a potential mechanism for inhibition of neurite outgrowth by MAG.

Highlights

Myelin-associated glycoprotein (MAG) is expressed on myelinating glia and inhibits neurite outgrowth from post-natal neurons
We show that mutation of this residue reduces the potency of MAG inhibitory activity but that residual activity is a result of carbohydrate recognition
We show that MAG binds both gangliosides and that both are expressed on the surface of MAGresponsive neurons

Summary

A MECHANISM FOR NEURITE OUTGROWTH INHIBITION*

MAG has a sialic acid binding site in its N-terminal domain and binds to specific sialylated glycans and gangliosides present on the surface of neurons, but the significance of these interactions in the effect of MAG on neurite outgrowth is unclear. Antibody cross-linking of cell surface GT1b, but not GD1a, mimics the effect of MAG, in that neurite outgrowth is inhibited through activation of Rho kinase. Mutation of arginine 118, an amino acid which is predicted to form hydrogen bonds with the carboxylate group of sialic acid [21], failed to inactivate the protein [29] This led to the suggestion that a second, sialic acid-independent, site on MAG interacts with an unknown counter-receptor on neurons, triggering the intracellular signaling cascade leading to inhibition of neurite outgrowth [29]. These results strongly suggest that GT1b represents a potential receptor for MAG, mediating inhibition of neurite outgrowth

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