Modulation of agrin binding and activity by the CT and related carbohydrate antigens.
Agrin is a nerve-derived signal that is essential for the proper organization of postsynaptic acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction. It is likely that carbohydrates play a significant role in regulating agrin activity, as agrin binds multiple glycan structures and is itself a highly glycosylated protein. Here we provide support for this contention by showing that agrin can be modified with the CT antigen, a carbohydrate structure expressed at the neuromuscular junction, and by describing the resulting changes in agrin binding to neoglycoconjugates and cultured myotubes, as well as changes in agrin-dependent AChR clustering. Glycosylation of agrin with the CT antigen required the mucin domain and the dystroglycan/heparin-binding domain. The presence of the mucin domain lowered agrin binding to several N-acetyllactosaminyl-containing saccharides and C2 myotubes and lowered agrin activity in AChR clustering. Glycosylation of agrin with the CT antigen, by contrast, increased agrin binding to myotubes and potentiated its AChR clustering activity at subsaturating concentrations. Last, sialylated and nonsialylated variants of N-acetyllactosamine differentially modulated AChR clustering and agrin activity, and these changes correlated with the ability of MuSK, an agrin-stimulated kinase, to bind to these structures. These experiments demonstrate that the glycosylation state of agrin affects its activity and suggest a role for the CT antigen in modulating agrin function.
Xia
Bing
B
Department of Neuroscience, Glycobiology Research and Training Center, University of California at San Diego School of Medicine, La Jolla 92093-0691, USA.
Martin
Paul T
PT
D018171:Agrin / Q000378:metabolism*; D000818:Animals; D000954:Antigens, Surface / Q000378:metabolism*; D019556:COS Cells; D002454:Cell Differentiation / Q000502:physiology*; D002490:Central Nervous System / Q000254:growth & development / Q000378:metabolism; D006001:Glycoconjugates / Q000378:metabolism; D006031:Glycosylation; D008562:Membrane Glycoproteins / Q000378:metabolism*; D009046:Motor Neurons / Q000166:cytology / Q000378:metabolism; D009077:Mucins / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D009469:Neuromuscular Junction / Q000166:cytology / Q000254:growth & development* / Q000378:metabolism; D009473:Neuronal Plasticity / Q000502:physiology*; D017729:Presynaptic Terminals / Q000378:metabolism / Q000648:ultrastructure; D011485:Protein Binding / Q000502:physiology*; D017434:Protein Structure, Tertiary / Q000502:physiology; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism; D011950:Receptors, Cholinergic / Q000378:metabolism
D018171:Agrin; D000954:Antigens, Surface; D006001:Glycoconjugates; D008562:Membrane Glycoproteins; D009077:Mucins; D011950:Receptors, Cholinergic; C095714:MUSK protein, human; D020794:Receptor Protein-Tyrosine Kinases