A synapse-specific carbohydrate at the neuromuscular junction: Association with both acetylcholinesterase and a glycolipid

Abstract

With the aim of investigating the roles of carbohydrates in synapse formation, we have characterized a synapse-specific saccharide at the vertebrate neuromuscular junction. Two lectins of similar specificity (Dolichos biflorus agglutinin, DBA, and Vicia villosa-B4 agglutinin, VVA-B4) stain synaptic but not extrasynaptic regions of the rat muscle fiber surface and thus define a synapse-specific carbohydrate. Using these and other probes, we show that the carbohydrate moiety concentrated at the neuromuscular junction resembles N-acetylgalactosamine (GalNAc) linked in the β-anomeric form to the termini of oligosaccharides. VVA-B4 also selectively stains neuromuscular junctions in human, mouse, rabbit, guinea pig, chick, frog, axolotl, snake, fish, and lamprey muscles, a phylogenetic conservatism that suggests a synapse-related role of GalNAc β-terminal saccharides. AChE from muscle binds to DBA- and VVA-B4-agarose, and is thereby identified as a glycoconjugate bearing the synapse-specific carbohydrate. Assay of AChE isoforms reveals that asymmetric, collagen-tailed forms, known to be highly concentrated at the rat neuromuscular junction, bind DBA and VVA-B4, while globular forms, which are more widely distributed, do not. A second class of GalNAc-bearing glycoconjugates is demonstrable immunohistochemically with monoclonal antibodies to stage-specific embryonic antigen (SSEA)-3 (Shevinsky et al., 1982) and GM2 (Natoli et al., 1986), which recognize GalNAc-bearing glycolipids. These antibodies selectively stain neuromuscular junctions, where they recognize glycolipid-like molecules that bind VVA-B4 but are distinguishable from AChE. The association of a synapse-specific carbohydrate with at least 2 different synapse-specific molecules raises the possibility that the former plays a role in determining a property that the latter share, such as concentration at the synapse.