Advances in cell surface glycoengineering reveal biological function

Abstract

Cell surface glycans are critical mediators of cell-cell, cell-ligand, and cell-pathogen interactions. By controlling the set of glycans displayed on the surface of a cell, it is possible to gain insight into the biological functions of glycans. Moreover, control of glycan expression can be used to direct cellular behavior. While genetic approaches to manipulate glycosyltransferase gene expression are available, their utility in glycan engineering has limitations due to the combinatorial nature of glycan biosynthesis and the functional redundancy of glycosyltransferase genes. Biochemical and chemical strategies offer valuable complements to these genetic approaches, notably by enabling introduction of unnatural functionalities, such as fluorophores, into cell surface glycans. Here, we describe some of the most recent developments in glycoengineering of cell surfaces, with an emphasis on strategies that employ novel chemical reagents. We highlight key examples of how these advances in cell surface glycan engineering enable study of cell surface glycans and their function. Exciting new technologies include synthetic lipid-glycans, new chemical reporters for metabolic oligosaccharide engineering to allow tandem and in vivo labeling of glycans, improved chemical and enzymatic methods for glycoproteomics, and metabolic glycosyltransferase inhibitors. Many chemical and biochemical reagents for glycan engineering are commercially available, facilitating their adoption by the biological community.

Keywords: glycan labeling; glycoproteomics; glycosyltransferases; metabolic oligosaccharide engineering; metabolism.

Fig. 1.

Cell surface display of defined glycans reveals glycan function and dictates cell fate. (A) Glycans linked to a hydrophobic anchor can either be directly added to cells of interest or delivered via liposomes. (B) Multiple approaches allow target cells to be engineered to display a bioorthogonal functional group. Glycans functionalized with a cognate functional group react with the target cells, yielding cell surface display of the desired glycan structures. This figure is available in black and white in print and in color at Glycobiology online.

Fig. 2.

Advances in MOE. (A) Introduction of both novel and established chemical reporters enables applications such as tandem labeling, in vivo imaging of inner organs, and identification of a glycans' binding partners. (B) A ManNAc derivative that carries an azide at the C4-position is incorporated exclusively into GalNAc-type O-linked glycans and thereby enables their study. This figure is available in black and white in print and in color at Glycobiology online.

Fig. 3.

Selective chemical labeling of cell surface glycans. Aldehydes are formed on sialic acid residues by mild periodate treatment (PAL; A) or on galactose residues by galactose oxidase treatment (GAL; B). Subsequent labeling of oxidized glycans with biotin or flurorescent reporters is performed by aniline-catalyzed oxime ligation. This figure is available in black and white in print and in color at Glycobiology online.

Fig. 4.

Selective enzymatic labeling of cell surface glycans. (A) Terminal LacNAc residues are selectively tagged using an α1–3-fucosyl transferase. (B) Glycans containing α1–2 fucose can be selectively labeled with Gal or GalNAc analogs using a bacterial glycosyltransferase, BgtA. (C) Non-sialylated N-linked glycans and O-linked glycans can be selectively labeled with a sialic acid analog using a sialyltransferase. Prior neuraminidase treatment makes additional glycans accessible for labeling. This figure is available in black and white in print and in color at Glycobiology online.