Mucin-type O-glycans enrich in white matter tracts

Protein O-glycosylation, the enzymatic attachment of carbohydrates to serine/threonine/tyrosine residues, is a large group of essential post-translational modifications involved in brain development and disease. However, studies of the most abundant O-glycan class in the mammalian brain, termed mucins or O-GalNAc glycans, are limited. Here, we determined the cellular specificity, spatial distribution, and protein carriers of O-GalNAc glycans in the mouse brain. Genetic inhibition of the O-GalNAc pathway in neurons or astrocytes had minimal effect on total O-GalNAc levels in the brain and failed to replicate the severe neurologic phenotype seen in rodent models and human congenital disorders of O-GalNAc synthesis. Further investigations revealed a surprising enrichment of O-GalNAc structures in white matter tracts and a spatial distribution distinct from other major carbohydrate structures in the brain including N-glycans. Using glycoproteomics, O-GalNAc glycans were identified on secretory proteins expressed in multiple cell types involved in brain development, synapse organization, and the extracellular matrix, including classical carriers of O-mannose glycans and proteoglycans. These findings highlight an overlooked aspect of O-GalNAc biology in white matter tracts of the brain and will inform future studies of development and disease.