Regulation of Intracellular Signaling by Extracellular Glycan Remodeling

Abstract

The plasma membrane of eukaryotic cells is coated with carbohydrates. By virtue of their extracellular position and recognizable chemical features, cell surface glycans mediate many receptor-ligand interactions. Recently, mammalian extracellular hydrolytic enzymes have been shown to modify the structure of cell surface glycans and consequently, alter their binding properties. These cell surface glycan remodeling events can cause rapid changes in critical signal transduction phenomena. This review highlights recent studies on the roles of eukaryotic extracellular sialidases, sulfatases, and a deacetylase in regulation of intracellular signaling. We also describe possible therapies that target extracellular glycan remodeling processes and discuss the potential for new discoveries in this area.The glycocalyx stands between extracellular signals and intracellular responses. But the glycoproteins, glycolipids and proteoglycans that comprise the glycocalyx are not simply a barrier; rather, they serve essential roles in the transduction of signals from the outside to the inside of cells. Indeed, most cell surface receptors are glycosylated and many specifically recognize glycans that are attached to their ligands. Cell surface and extracellular glycans have been demonstrated to play critical roles in the control and modulation of a variety of signal transduction pathways (1-4). For example, in embryonic development, signaling through the Notch receptor requires that the receptor be modified with O-fucose glycans (5). If these glycans are absent, gestational death occurs. In the adult nervous system, the myelin-associated glycoprotein (MAG) binds to glycoproteins and glycolipids on axons, forming signaling complexes that inhibit axonal outgrowth (6). And in the immune system, specific cell surface glycans are essential components of the signal transduction pathways that lead to processes such as B-cell receptor activation and T-cell apoptosis (7,8). Glycosylation also regulates intracellular signal transduction events: for example, the addition of O-linked Nacetylglucosamine (O-GlcNAc) to histone lysine methyl transferase MLL5 activates this enzyme, causing it to methylate histone H3 and thereby leading to cell lineage determination (9).As more signaling roles for glycans are identified, an essential task is to understand the mechanisms that regulate the glycosylation state of a cell or molecule (Figure 1). Most glycans are assembled in the endoplasmic reticulum (ER) and Golgi through the coordinate action of many membrane-associated glycosyltransferases and other glycan-modifying enzymes, such as sulfotransferases and epimerases. ER-and Golgi-resident glycosyltransferases require nucleotide-sugar donors, such as uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc), guanidine diphosphate-fucose (GDP-fucose), and cytidine monophosphate-sialic acids (CMPsialic acids), that are synthesized in the cytoplasm or nucleus. Once assembled in the secretory *To whom correspondence should be addressed (Jennifer.Kohler@UTSou...

N- and O-Glycosylation in the Murine Synaptosome

“…Although ER/Golgi-derived glycosylation is one of the most frequently occurring classes of PTMs (6), these extracellular modifications continue to be a major challenge because of technical difficulties both in their structural characterization and in deciphering their biological roles. However, there is a renewed interest in the characterization of glycosylation driven by the production of protein pharmaceuticals as well as by the search for biomarkers and by new findings about the function(s) of glycosylation (7)(8)(9)(10)(11)(12).…”

A Novel Method for the Simultaneous Enrichment, Identification, and Quantification of Phosphopeptides and Sialylated Glycopeptides Applied to a Temporal Profile of Mouse Brain Development

“…The brain structures are reorganized with the extension of axons and dendrites to communicate via synaptic terminal interactions (1,2). These molecular interactions are governed by cell surface receptors that are often post-translationally modified with both N-linked glycans and phosphate groups, and studies have suggested that extracellular glycans play vital roles in the regulation of signal transduction pathways (3). For example, the myelin-associated glycoprotein (MAG) binds to cell surface glyco-conjugates GD1a, GT1b and Nogo receptors to form signaling complexes that inhibit axon outgrowth, whereas inhibition of Rho kinase reverses this process in a number of nerve cell types (4).…”

Sensitive and Structure-Informative <i>N</i>-Glycosylation Analysis by MALDI-MS; Ionization, Fragmentation, and Derivatization

“…In addition, it has been reported that changes in the extent of sialylation on glycoproteins as a consequence of various diseases can modify downstream intracellular signaling. 60) is indicates the importance of elucidating glycan structures, including sialylation patterns (i.e., the number of sialic acid residues and their linkage types). However, the presence of sialic acid residues on glycans causes various analytical problems in MS as follows: (1) ey confer a negative charge on the glycans and thus generally decrease their ionization e ciency, biasing mass spectrometric quanti cation; (2) Due to their instability, sialic acids are readily lost in mass spectrometric steps (i.e., in-and post-source decay) as well as pre-treatment steps.…”