MYELIN-ASSOCIATED GLYCOPROTEIN INTERACTS WITH NEURONS VIA A SIALIC-ACID BINDING-SITE AT ARG118 AND A DISTINCT NEURITE INHIBITION SITE

Inhibitory components in myelin are largely responsible for the lack o f regeneration in the mammalian CNS. Myelin-associated glycoprotein (M AG), a sialic acid binding protein and a component of myelin, is a pot ent inhibitor of neurite outgrowth from a variety of neurons both in v itro and in vivo. Here, we show that MAG's sialic acid binding site is distinct from its neurite inhibitory activity. Alone, sialic acid-dep endent binding of MAG to neurons is insufficient to effect inhibition of axonal growth. Thus, while soluble MAG-Fc (MAG extracellular domain fused to Fc), a truncated form of MAG-Fc missing Ig-domains 4 and 5, MAG(d1-3)-Fc, and another sialic acid binding protein, sialoadhesin, e ach bind to neurons in a sialic acid-dependent manner, only full-lengt h MAG-Fc inhibits neurite outgrowth. These results suggest that a seco nd site must exist on MAG which elicits this response. Consistent with this model, mutation of arginine 118 (R118) in MAG to either alanine or aspartate abolishes its sialic acid-dependent binding. However, whe n expressed at the surface of either CHO or Schwann cells, R118-mutate d MAG retains the ability to inhibit axonal outgrowth. Hence, MAG has two recognition sites for neurons, the sialic acid binding site at R11 8 and a distinct inhibition site which is absent from the first three Ig domains.