Besides the ill-fated Keto diet you promised yourself you would go on last New Year's, when was the last time you thought about sugar and your health? We as scientists have a tendency to think of biology in terms of the the major players: nucleic acids and proteins. In fact, carbohydrate-derived macromolecules called glycans have long been recognized for their role in immunology, and recent discoveries are sparking renewed interest in glycosylation as an immune modulator. Today, we explore some advances at the intersection of glycobiology and immunology.

What are glycans? 

At the most basic level, glycans are the the union of saccharide chains to an underlying protein or lipid scaffold. Amazingly, glycans can be found in dense layers covering the surface of all natural cells. It is thought that the remarkable ubiquity of cell surface glycosylation that exists in nature is a consequence of its ability to shield a host cell from pathogens that would otherwise quickly adapt to recognize cell surface proteins (1). Why might this be? For one, glycans are a common point of attachment for invading pathogens but can be rapidly remodeled without sacrificing underlying protein function. Compared to the relatively limited diversity of linear structures achievable with any given quantity of amino acids, orders of magnitude more structures are possible with the same number of monosaccharides (2).

Glycan Synthesis 

The staggering diversity of glycan construction stems from the branching nature of glycan architecture (depicted above), combined with the variety of individual monosaccharide units and the conformation of their attachment to each other. In the current understanding of glycan synthesis, activated nucleotide sugar donors are synthesized in the nucleus or cytoplasm. They are then actively transported into the lumen of the Golgi and ER and added to glycan chains on newly synthesized proteins and lipids by ER- and Golgi-resident glycosyltransferases. The nine monosaccharides that form the basis for mammalian glycosylation are linked together via glycosidic bonds, and typically take the form of N-glycans, O-glycans, and glycosaminoglycans in the context of protein glycosylation. N-linked glycans are found attached to the asparagine residues of proteins, specifically within an Asn-X-Ser/Thr motif. O-glycans are a little less picky about their attachment points, and often link to either serine or threonine residues. Glycosaminoglycans also attach to serine or threonine, but differ from O-glycans in that they are composed of linear chains of repeating di-saccharides rather than branched structures.

Glycan Regulation of Function 

How do glycans regulate the function of cells? The answer seems to lie in a combination of the below factors:

1. Size
2. Location
3. Charge

Glycans and Immunity 

Through the mechanisms listed above, glycans impact immunity by regulating signal transduction and receptor activation, protein folding, and cell adhesion. One of the best-known examples of this is the role of the Sialyl-LewisX (sLeX) oligosaccharide in leukocyte tethering and rolling. sLeX, most commonly found on O-glycans, is composed of sialic acid, galactose, N-acetylglucosamine (GlcNAc), and fucose. As a part of the multi-step process preceding extravasation to inflamed tissue, circulating leukocytes must first tether themselves to the vascular endothelium via interaction with lectin-like adhesion molecules called selectins. Selectins facilitate tethering and low-velocity rolling of leukocytes that is necessary for subsequent firm attachment and extravasation. P-selectin is rapidly expressed on the surface of activated endothelial cells, where it binds its primary ligand PSGL-1 present on the surface of hematopoietic cells. sLeX is necessary for P-selectin binding to PSGL-1 (4). PSGL-1 binds to P-selectin with strong affinity but quick association and disassociation rates, which explain why this interaction is perfect for slowing down quickly moving leukocytes in circulation (5). 

Glycosylation can also modulate signal transduction by regulating homotypic receptor and ligand interactions. For example, clustering of T-cell receptors (TCR) is necessary for T-cell activation. Deficiency of an enzyme responsible for the construction of N-glycans was found to correlate with increased clustering of TCR and downstream signaling pathways (6). The enzyme, Mgat5, initiates GlcNAc β1,6 branching on TCR N-glycans. The loss of Mgat5 resulted in fewer ligands for galectin-3, a lectin known to maintain the spatial organization of TCR. Interestingly, overexpression of Mgat5 is also associated with increased epidermal growth factor signaling (7). In this case, the matrix created between galectin-3 and Mgat5-glycosylated receptor N-glycans increased cell-surface retention of the receptor. 

Another example of famous sugars are the blood group antigens, discovered in the early 20th century by Karl Landsteiner and colleagues. Many people are familiar with these as the molecules that determine one’s blood type. The ABO blood group antigens (along with the Rh group) are the major determinants of blood types, producing either A, B, AB, or O types. These antigens are formed by the ABO gene, which encode a different glycosyltransferase depending on the allele. The A allele encodes an α1-3GalNAc transferase and the B allele encodes an α1-3Gal transferase, both of which modify underlying H Type-1, 2, 3, or 4 determinants on erythrocyte glycoproteins. O alleles encode an inactive glycosyltransferase, which is why people with blood type O lack the A or B antigen and are termed "universal donors."

Conclusion

References

1. A Varki. 2011. Cold Spring Harb Perspect Biol. 3(6).
2. Schnaar R, Gerardy-Schahn, R, and Hildebrandt, H. 2014. Physiol Rev. 94: 461-518.
3. Ohtsubo K and Marth J. 2006. Cell. 126(5): 855-867.
4. Zhou Q, et al. 1991. JCB. 115(2): 557-64.
5. Mehta P, Cummings RD, and McEver RP. 1998. JBC. 273(49): 32506-13.
6. Demetriou M, et al. 2001. Nature. 409(6821): 733-9.
7. Partridge EA, et al. 2004. Science. 306(5693): 120-4.

Contributed by Christopher Dougher, Ph.D.