Unique Glycans in Synaptic Glycoproteins in Mouse Brain

The synapse is an essential connection between neuronal cells in which the membrane and secreted glycoproteins regulate neurotransmission. The post-translational modifications of glycoproteins with carbohydrates, although essential for their functions as well as their specific localization, are not well understood. Oddly, whereas galactose addition to glycoproteins is required for neuronal functions, galactosylation is severely restricted for Asn-linked on N-glycans in the brain, and genetic evidence highlights the important roles of galactose in brain functions and development. To explore this novel glycosylation, we exploited an orthogonal technology in which a biotinylated sialic acid derivative (CMP-biotin-Sia) is transferred to terminally galactosylated proteins by a recombinant sialyltransferase (rST6Gal1). This approach allowed us to identify the carrier proteins as well as their localization on brain sections. Immunohistochemical analysis of the biotinylated glycoproteins in brain sections demonstrates that they are largely positioned in the pre- and postsynaptic membranes. Consistent with this positioning, glycoproteomic analyses of the labeled glycoproteins identified a number of them that are involved in synaptic function, cell adhesion, and extracellular matrix interactions. The discovery of these galactosylated N-glycoproteins and their relative confinement to synapses provide novel insights into the unusual and specific nature of protein glycosylation in the brain.