Vinson M, Strijbos P J, Rowles A, Facci L, Moore S E, Simmons D L, Walsh F S
Neurology Centre of Excellence for Drug Discovery, GlaxoSmithKline, New Frontiers Science Park North, Third Ave., Harlow, Essex CM19 5AW, United Kingdom.
J Biol Chem. 2001 Jun 8;276(23):20280-5. doi: 10.1074/jbc.M100345200. Epub 2001 Mar 6.
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.
髓鞘相关糖蛋白（MAG）在髓鞘形成的神经胶质细胞上表达，并抑制出生后神经元的神经突生长。MAG在其N端结构域有一个唾液酸结合位点，可与神经元表面存在的特定唾液酸化聚糖和神经节苷脂结合，但这些相互作用在MAG对神经突生长影响中的意义尚不清楚。在此，我们提供证据表明，唾液酸化聚糖的识别对于MAG抑制神经突生长至关重要。已知MAG上的精氨酸118与唾液酸形成关键接触。我们表明，该残基的突变降低了MAG抑制活性的效力，但残余活性也是碳水化合物识别的结果。然后，我们继续研究神经节苷脂GT1b和GD1a作为MAG候选受体。我们表明，MAG特异性结合这两种神经节苷脂，并且它们都在对MAG有反应的神经元表面表达。此外，细胞表面GT1b而非GD1a的抗体交联模拟了MAG的作用，即通过激活Rho激酶抑制神经突生长。这些数据强烈表明，与神经元细胞表面的GT1b相互作用是MAG抑制神经突生长的一种潜在机制。
