Targeting Selectins, Siglecs and Mammalian Glycans
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REVIEWS The clinical impact of glycobiology: targeting selectins, Siglecs and mammalian glycans Benjamin A. H. Smith 1 and Carolyn R. Bertozzi1,2,3 ✉ Abstract | Carbohydrates — namely glycans — decorate every cell in the human body and most secreted proteins. Advances in genomics, glycoproteomics and tools from chemical biology have made glycobiology more tractable and understandable. Dysregulated glycosylation plays a major role in disease processes from immune evasion to cognition, sparking research that aims to target glycans for therapeutic benefit. The field is now poised for a boom in drug development. As a harbinger of this activity, glycobiology has already produced several drugs that have improved human health or are currently being translated to the clinic. Focusing on three areas — selectins, Siglecs and glycan- targeted antibodies — this Review aims to tell the stories behind therapies inspired by glycans and to outline how the lessons learned from these approaches are paving the way for future glycobiology-focused therapeutics. Glycoproteins Glycans are involved in fundamental aspects of cell and 20 different ways of linking together glucose and galac- Proteins that are covalently organismal biology, such as the receptor- mediated cell tose in their ring forms through a glycosidic bond, 19 of conjugated to one or more to cell interactions that underlie both normal and patho- which do not make lactose. Additional complexity arises glycans. These glycans may logical processes. Indeed, the dense layer of glycans on from modification of glycans by sulfation, methylation, be either N- linked (connected to asparagine) or O- linked the cell surface (the glycocalyx) can extend more than phosphorylation, acetylation and O-acylation. 1 (connected to serine or 30 nm from the plasma membrane on some cells . Cell Glycosylation has three broad functions. First, some threonine). surface proteins are therefore embedded in a matrix of glycans form structures with unique physical properties. glycans. Second, glycans can regulate the function or properties The varied functions of glycans are matched by their of the entity to which they are attached, for instance by diverse structures. Glycans can be conjugated to proteins controlling protein stability or receptor dimerization. (to form glycoproteins, proteoglycans and glycosylphos- Last, certain glycans are themselves ligands for lectins, phatidylinositol (GPI)-anchored proteins) and lipids (to which are carbohydrate-specific receptors. form glycolipids), or they can be secreted without conju- Because glycans are essential for organism health, gation to other macromolecules (in the form of glycos- defects in glycosylation are important contribu- aminoglycans such as hyaluronan). In humans, glycans tors to human disease. However, the development of are primarily constructed from ten monosaccharides: glycan- targeted therapies has been hindered by many glucose (Glc), galactose (Gal), N- acetylglucosamine factors, beginning with our lack of tools for understand- (GlcNAc), N- acetylgalactosamine (GalNAc), fucose ing basic glycobiology. Research in the field hinges on (Fuc), xylose (Xyl), sialic acid (Neu5Ac), glucuronic 1Department of Chemical accurate methods for identifying and quantifying gly- & Systems Biology and acid (GlcA), mannose (Man) and iduronic acid (IdoA). cans in a sample. These profiling experiments typically ChEM- H, Stanford School of The assembly of these monosaccharides into glycans is rely on mass spectrometry, fluorophore- conjugated Medicine, Stanford, CA, USA. performed by enzymes associated with the endoplasmic lectins or antibodies. For the few glycan species with 2Department of Chemistry, reticulum and Golgi apparatus. Monosaccharides are high- affinity anti- glycan antibodies, the pace of dis- Stanford University, Stanford, linked together through a glycosidic bond between the covery is higher than for those without such reagents. CA, USA. anomeric carbon of one sugar and a hydroxyl group of However, relatively few antibodies against defined gly- 3 Howard Hughes Medical the other. The orientation of the glycosidic bond relative cans are available. Mass spectrometry- based glycomics Institute, Stanford University, Stanford, CA, USA. to the anomeric carbon (α versus β) affects the overall provides an inventory of glycans present in the sample, ✉e- mail: shape of the glycan. Therefore, the notation for lactose, but requires liberating glycans from their underlying [email protected] Galβ1–4Glc, for example, refers to a galactose linked scaffold. As a consequence, information is limited about https://doi.org/10.1038/ though a β- glycosidic bond to the hydroxyl group on which glycan is attached to the particular glycosites on s41573-020-00093-1 C4 of glucose. Considering these factors alone, there are any given protein or lipid. Glycoproteomics is becoming NATURE REVIEWS | DRUG DISCOVERY VOLUME 20 | MARCH 2021 | 217 REVIEWS Proteoglycans a viable alternative as technologies for sample prepara- Selectins Glycoconjugates comprising tion, database searching and data processing continue The selectins are a family of calcium-dependent (C-type) a core protein decorated with to improve2,3. lectins best known for their role in mediating immune glycosaminoglycans, which are Experiments aimed at identifying the set of ligands cell adhesion to the endothelium to facilitate entry to long, linear chains of repeating disaccharides that are for a biologically relevant lectin are complicated by the secondary lymphoid organs and sites of inflammation. frequently sulfated. The sugars nature of the interactions between lectins and their gly- Therapies targeting selectin–ligand interactions have constituting this repeating unit cosylated ligands. First, monovalent interactions tend to been inspired by the importance of selectins in medi- are characteristic of different be of low affinity; multivalent presentation of both lec- ating cell adhesion, and have been investigated in sickle glycosaminoglycans. tin and ligand is often needed to achieve physiologically cell disease, cancer cell metastasis and bone marrow For instance, repeating 4 N- acetylgalactosamine relevant binding affinities . Second, lectins may have transplantation. (GalNAc) and glucuronic acid binding affinities that are not only higher but directed The selectin family comprises three members, named (GlcA) distinguish chondroitin to structurally different glycans when those glycans are after their expression patterns: those expressed on plate- sulfate, whereas heparan presented on a multivalent scaffold as compared with lets (P- selectin), on endothelial cells (E-selectin) and on sulfate begins as repeating 5 N- acetylglucosamine (GlcNAc) a monovalent interaction in solution . Third, in vitro leukocytes (L- selectin). These three selectins also dif- and GlcA before undergoing assays may not capture in vivo complexity; indeed, fer in biological activities and preferred ligands (Fig. 1). heavy modifications, such as shear stress is required to observe catch bonds, which are L- selectin is constitutively expressed on all circulating epimerization to IdoA and characteristic of selectin–ligand interactions6. Last, the leukocytes and is shed from the cell surface. E- selectin is sulfation. binding epitope on the ligand may contain both glycan constitutively expressed on endothelial cells in postcapil- Catch bonds and scaffold protein or lipid components. Therefore, lary venules of the bone marrow and skin; expression Bonds that prolong their screening fragments of glycans, proteins and lipids on the endothelium in other organs requires exposure lifetime in response to independently of one another for binding to a particu- to inflammatory stimuli such as tumour necrosis factor increased tensile forces. lar lectin is unlikely to identify relevant ligands. This (TNF), interleukin 1β (IL-1β) and lipopolysaccharide By contrast, the lifetime of a slip bond decreases in is an important caveat to glycan array technology, in (LPS). P- selectin is expressed by endothelial cells and 10 response to higher tension. which glycans are printed on glass slides and binding activated platelets . is detected by incubation with fluorescently labelled Among the best described ligands for the selectins are Leukocyte extravasation lectins7. derivatives of the fucosylated and sialylated tetrasaccha- The process by which cells Glycan synthesis is also a non- templated process, rides sialyl Lewisx (sLex, Neu5Acα2–3Galβ1–4(Fucα1–3) move from the circulation, a a across the blood vessel wall meaning that glycan sequences are not directly coded in GlcNAc) and sialyl Lewis (sLe , Neu5Acα2–3Galβ1– and into tissue parenchyma. the genome. Instead, glycans are produced by the coor- 3(Fucα1–4)GlcNAc). The context in which these gly- Extravasation is a stepwise dinated activity of hundreds of biosynthetic enzymes. cans are presented alters their affinity for any given process beginning with rolling Therefore, a particular glycan cannot easily be geneti- selectin. For instance, the primary ligand for P- selectin adhesion, then tight binding of leukocytes to the endothelium cally deleted or altered in order to explore its function. is P- selectin glycoprotein ligand 1 (PSGL1), a homodi- x 11,12 and, finally, diapedesis, where At best, pathways for glycan biosynthesis can be inhib- meric mucin decorated with sLe (REFS ), but P-selectin cells cross the endothelial ited or engineered, and synthetic glycans on polymeric also has high- affinity interactions