O‐Glycan recognition and function in mice and humans

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Glycans function both as direct recognition molecules, where they are bound by a wide variety of glycan‐binding proteins (GBPs), including animal cell receptors, microbial adhesins lectins and toxins, and indirectly glycans function in glycoproteins to facilitate intra‐ and intermolecular interactions that maintain homeostasis and biological integrity. To explore both direct and indirect functions of glycans we have engineered mutations in biosynthetic pathways, most notably in regard to O‐glycan biosynthesis (O‐GalNAc glycans). Normal synthesis of such O‐glycans in animals requires the action of the T‐synthase, a b3‐galactosyltransferase that generates the core 1 O‐glycan Galb1‐3GalNAca1‐Ser/Thr from the Tn antigen precursor GalNAca1‐Ser/Thr. Such normal extended O‐glycans, including those with the core 2 O‐glycan structure and sialyl Lewis x antigen, are recognized by key GBPs, including P‐selectin and L‐selectin. Such specific recognition of O‐glycans by P‐selectin, CLEC2, and other GBPs, directly regulated many cellular interactions and signaling pathways important in development, morphogenesis and organ development. In molecular and biochemical studies we have explored such recognition by generating glycopeptides with specific modifications, including O‐glycans and sulfated tyrosine residues. In terms of additional functions of O‐glycans, we have exploited our discovery that the T‐synthase requires a specific protein chaperone, termed Cosmc, encoded on the X‐chromosome. Cosmc, a resident protein in the endoplasmic reticulum, directly interacts with the newly synthesized T‐synthase to facilitate folding to the active form of the enzyme, that subsequently moves to its functional location in the Golgi apparatus. Thus, Cosmc is the key regulator of Tn antigen expression, and loss of Cosmc, through epigenetic silencing, gene mutations and other pathways, and acquired changes in Cosmc function cause Tn expression in many human tumors and tumor cell lines and in Tn syndrome patients. Targeted deletion of Cosmc leads to loss of normal O‐glycans, expression of the Tn and sialyl Tn antigens, and multiple pathological consequences. In this presentation the loss of Cosmc has been explored in the gastrointestinal tract, where it leads to pathology, chronic inflammation, and changes in the gut microbiome, and in other cell types, such as T cells and B cells, where loss of Cosmc alters cellular maturation and functionalities. Our studies are revealing the multiple ways in which O‐glycans in humans, animals, and other organisms interact within the larger protein‐glycan interactome to regulate biological processes.Support or Funding InformationThis work was supported by NIH grants U01CA168930 and P41GM103694 to R.D.C.