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 of regeneration in the mammalian CNS. Myelin-associated glycoprotein (MAG), a sialic acid binding protein and a component of myelin, is a potent inhibitor of neurite outgrowth from a variety of neurons both in vitro and in vivo. Here, we show that MAG's sialic acid binding site is distinct from its neurite inhibitory activity. Alone, sialic acid-dependent 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, each bind to neurons in a sialic acid- dependent manner, only full-length MAG-Fc inhibits neurite outgrowth. These results suggest that a second 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, when expressed at the surface of either CHO or Schwann cells, R118-mutated MAG retains the ability to inhibit axonal outgrowth. Hence, MAG has two recognition sites for neurons, the sialic acid binding site at R118 and a distinct inhibition site which is absent from the first three Ig domains.
Tang
S
S
Department of Biological Sciences, Hunter College of the City University of New York 10021, USA.
Shen
Y J
YJ
DeBellard
M E
ME
Mukhopadhyay
G
G
Salzer
J L
JL
Crocker
P R
PR
Filbin
M T
MT
D000595:Amino Acid Sequence; D000818:Animals; D001120:Arginine / Q000378:metabolism; D001665:Binding Sites / Q000502:physiology; D016466:CHO Cells / Q000502:physiology; D002448:Cell Adhesion / Q000502:physiology; D006224:Cricetinae; D008969:Molecular Sequence Data; D016296:Mutagenesis / Q000502:physiology; D018992:Myelin-Associated Glycoprotein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019158:N-Acetylneuraminic Acid / Q000737:chemistry / Q000378:metabolism*; D016501:Neurites / Q000737:chemistry* / Q000502:physiology; D009474:Neurons / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D017434:Protein Structure, Tertiary; D012583:Schwann Cells / Q000166:cytology / Q000502:physiology; D014162:Transfection
D018992:Myelin-Associated Glycoprotein; D001120:Arginine; D019158:N-Acetylneuraminic Acid