Somataglycan-S: a neuronal surface proteoglycan defines the spinocerebellar system

The formation and maintenance of functionally specific neuronal networks may depend on specific proteoglycans localized to the surface membranes of a subset of neurons. Monoclonal antibody (MAb) 6A2 labeled a distinct subset of CNS neurons: the somas and proximal dendrites of cells making up the spinocerebellar and reticular systems. These pathways contribute to proprioceptive and exteroceptive functions. Ultrastructurally, MAb 6A2 immunoreactivity was distributed focally along the cell surface membranes and the adjacent extracellular space. On western blots of immunoaffinity-purified preparations from cerebellar homogenates, a major, broad band of approximately 400 kDa is labeled by MAb 6A2. Increased electrophoretic mobility of the purified antigen after digestion with chondroitinase ABC and keratanase suggests that the antigen is a proteoglycan bearing chondroitin sulfate and keratan sulfate glycosaminoglycans. Unsulfated N-acetyl-galactosamine residues linked to unsaturated uronic acid constituted the initial disaccharide in the chondroitin sulfate glycosaminoglycan chains. N- and O-linked oligosaccharides on the core protein were detected by the biotinylated lectins wheat germ agglutinin and Jacalin, respectively, and by MAb anti-HNK-1. Lyase and glycosidase digests result in a 280-kDa band. This proteoglycan, somataglycan-S, may provide a key to the role of glycoconjugates in determining neuronal diversity and system specificity.