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Chemical Regulation of Glycosylation Processes

Glycans (monosaccharides and oligosaccharides) and their conjugates (glycoproteins, glycolipids, and proteoglycans) are structurally diverse biomolecules that are involved in many biological processes of health and disease. The structural diversity of glycans and glycoconjugates is owed to their monosaccharide composition, anomeric state, glycosidic linkage, modification (phosphorylation, sulfation, acetylation, etc.) and aglycone (protein, lipid, etc.). These diverse structures are controlled by complex glycosylation processes in cells, which are mediated by various glycosyltransferases and glycosidases. Glycosylation processes can be chemically regulated by inhibition of glycosyltransferases or glycosidases with natural and synthetic molecules. Treatment of cells with inhibitors of these enzymes results in the production of glycans or glycoconjugates containing missing or altered glycan chains. This approach is highly useful for examining the potential functional role(s) of glycans and glycoconjugates in cells or tissues, and in biological processes of health and disease. Eventually, it will provide novel mechanisms for disease treatment. This review highlights recent developments in chemical regulation of glycosylation processes with specific targets including: inhibition of (1) Nglycosylation, (2) O-glycosylation, (3) O-linked GlcNAc glycosylation, (4) proteoglycan biosynthesis, (5) glycolipid biosynthesis, and (6) terminal glycosylation. The goal of this review is to provide researchers with more competent choices in their research and lay a foundation on which continued advancements can be made to promote further explorations in glycoscience and biomedical research and applications. A. Introduction Glycoconjugates are diverse biomacromolecule of proteins and/or lipids modified with glycans in cells, which include glycoproteins, proteoglycans, GPI-anchored proteins, and glycolipids (Fig. 1A). The majority of glycoconjugates are expressed on the cell surface, where they form a thick layer known as glycocalyx. Glycoconjugates are also secreted inside of the cells, tissues and biofluids in the body. Glycoconjugates participate in many key biological processes including cellular adhesion, migration, growth, differentiation, signal transduction, receptor activation, immune response modulation, quality control of protein folding, and hostpathogen interactions (1). However, the molecular mechanisms in most events with the glycoconjugates involved are still unclear. Mostly, the role(s) of glycans on a specific biomolecule cannot be explained. Therefore, there is a high demand for studying glycan and glycoconjugate structures and functions for both basic biological sciences and biomedical applications. Glycoconjugates are produced by the complex process of glycosylation, which is mediated by various glycosyltransferases and glycosidases (2). In mammalian cells, glycans are built up from ten monosaccharides—fucose (Fuc), galactose (Gal), N-acetylgalactosamine (GalNAc), glucose (Glc), N-acetylglucosamine (GlcNAc), glucuronic acid (GlcA), iduronic acid (IdoA), mannose (Man), sialic acid (Neu5Ac as the major sialic acid) and xylose (Xyl) (Fig. 1B). Except for IdoA, which forms from epimerization of GlcA, the other nine monosaccharides are mainly generated by the de novo pathway ultimately from glucose, but also by the salvage pathway that utilizes sugars from recycled glycans or from extracellular origins. Most of these monosaccharides are converted to nucleotides for glycosylation processes generating corresponding glycans and glycoconjugates (Fig. 1B). The de novo pathway is highly conserved across species and accounts for most mammalian nucleotides. Glycosylation processes are comprised of sequential enzymatic reactions that occur in different cellular compartments, such as the cytosol, endoplasmic reticulum (ER), Golgi apparatus, nucleus, and the plasma membrane (3). The biochemical and enzymatic basis of glycosylation has recently been extensively reviewed elsewhere (4–6), and thus is discussed briefly from the point of regulation of glycosylation in this review. Overall, five classes of glycosylation, including N-linked, O-linked, O-GlcNAclinked, GAG-linked, and glycolipid glycosylation are discussed for their biosynthesis and chemical regulation with inhibitors. Mostly, five major classes of glycosyltransferases that catalyze these glycosylation reactions are summarized in Table 1. The most common glycosyltransferases transfer sugar to an acceptor molecule using sugar nucleotide donors (e.g., UDP-Gal, GDP-Fuc, or CMP-Neu5Ac). Other types transfer sugar from a lipid-phosphoglycan donor, such as dolichol-phospho-mannose (or glucose) and dolichol-pyrophospho-oligosaccharide to acceptor molecules. Regulation/manipulation of glycosylation processes in cells MINIREVIEW doi: 10.4052/tigg.1306.1E Invited Review for Trends in Glycoscience and Glycotechnology (TIGG)