Research Highlights

O-glycan manufacture: No redundancies

O-glycans are saccharides linked to a serine or threonine residue on a glycoprotein. They function as a protective barrier coating epithelial cells, and as ligands for lymphocyte migration and other immune functions. Produced in the secretory pathway, O-glycan variations are distinguished by the monosaccharide arrangement forming the core. The core 1 O-glycan structure, a galactose molecule linked to an N-acetylgalactosamine (GalNAc), is modified by glycosyltransferases to generate core 2, core 3, or core 4 structures. Core 2 O-glycans are built by one of three core 2 1,6-N-acetylglucosaminyltransferases (C2GnTs), which add an N-acetylglucosamine (GlcNAc) to the GalNAc of the core 1 O-glycan. Mice deficient in one of these core 2 glycosyltransferases, C2GnT1, are unable to properly synthesize selectin ligands required for neutrophil recruitment and B cell homing. In Molecular and Cellular Biology, Stone et al. investigate why two other enzymes involved in core 2 O-glycan biosynthesis, C2GnT2 and C2GnT3, have also been evolutionarily conserved.

Unlike C2GnT1-deficient mice, mice lacking C2GnT2 or C2GnT3 continued to produce selectin ligands. However, the mucosal barrier of mice lacking C2GnT2 was defective, and was associated with increased susceptibility to experimentally-induced colitis. In addition, loss of C2GnT2 resulted in reduced levels of IgG and IgA antibodies, which implicate C2GnT2-mediated core 2 O-glycan synthesis in disease resistance.

Mice engineered to lack the third core 2 synthesis enzyme, C2GnT3, showed no change in immune profile, as was observed in C2GnT2-deficient mice. Nevertheless, C2GnT3 deficient mice behaved more aggressively than their wild-type counterparts. Further analysis linked expression of C2GnT3 with abundance of thyroxine, a circulating thyroid hormone that can influence aggression.

To determine whether these transferase enzymes are able to compensate for one another, Stone et al. generated mice lacking two of the three, or all three, C2GnTs. Although the functional consequences of disrupting C2GnT1, C2GnT2, and C2GnT3 expression varied widely, they found that all double and triple knockout mice were viable. The effects of the single deficiencies persisted in the triple knockout and the most significant additional phenotype was a change in liver function. Glycan structural analyses revealed the absence of all detectable core 2 O-glycans in the triple knockout, yet remarkable changes in other O-glycans also resulted, such as increased core 1 O-glycans, and surprisingly, increased O-mannosylation. These results indicate that, rather than serving redundant roles, these three core 2 glycosyltransferases serve unique functions.

This study explains why three different C2GnT glycosyltransferases have been conserved for what otherwise appears to be the same enzymatic function. As more glycan structures are defined in the context of normal and pathological states, appreciation for the role of O-glycosylation in disease prevention and maintenance of homeostasis is likely to grow.

Author: Heather Buschman