Therapeutically targeting protein-glycan interactions

Abstract

Glycosylation is the most common form of post-translational modifications by which oligosaccharide side chains are covalently attached to specific residues of the core protein. Especially O-linked glycan structures like the glycosaminoglycans were found to contribute significantly to many (patho-)biological processes like inflammation, coagulation, cancer and viral infections. Glycans exert their function by interacting with proteins thereby changing the structure of the interacting proteins and consequently modulating their function. Given the complex nature of cell-surface and extracellular matrix glycan structures, this therapeutic site has been neglected for a long time, the only exception being the antithrombin III-glycan interaction which has been successfully targeted by unfractionated and low-molecular weight heparins for many decades. Due to the recent breakthrough in the '-ome' sciences, among them proteomics and glycomics, protein-glycan interactions became more amenable for therapeutic approaches so that novel inhibitors of this interaction are currently in preclinical and clinical studies. An overview of current approaches, their advantages and disadvantages, is given and the promising potential of pharmacologically interfering with protein-glycan interactions is highlighted here.

Figure 1

Scheme of HS-biosynthesis. Displayed is the consecutive action of the various biosynthetic enzymes durin chain initiation, polymerization and modification (the recently proposed model of a so-called GAGosome, in which the biosynthetic steps occur in a parallelized way (Ledin et al., 2006), has not been considered in this figure). Enzymes involved in HS biosynthesis are: XYLT1, xylosyltransferase-1; XYLT2, xylosyltransferase-2; GalT1, galactosyltransferase-1; GalT2, galactosyltransferase-2; GlcAT1, glucuronosyltransferase-1; EXT1, exostosin-1; EXT2, exostosin-2; EXTL2, exostosin-like-2; EXTL3, exostosin-like-3; NDST1-4, N-deacetylase/N-sulfotransferase-1, -2, -3, -4; GLCE, D-glucuronyl C5-epimerase; 2OST, 2-O-sulfotransferase; 3OST1-6, 3-O-sulfotransferase-1, -2, -3A, -3B, -4, -5, -6; 6OST1-3, 6-O-sulfotransferase-1, -2, -3; SULF1, extracellular sulfatase Sulf-1; SULF2, extracellular sulfatase Sulf-2; HPSE, heparanase. Two typical results of taylored HS biosynthesis are given in the form of the FGF2- and the ATIII-specific binding sites. HS, heparan sulphate.

Figure 2

Cell surface of endothelial cells. In green, (trans-)membrane proteins are shown; branched and short red tubes represent N-linked glycans, long and unbranched red tubes represent O-linked (glycoasmino-)gycans; in blue, glycan-binding proteins are shown, different shades refer to different proteins and thus to hetero- or homo-oligomerization.

Figure 3

Different ways for therapeutic intervention in protein–GAG signalling. sequence-specific GAG – Oligo within a proteoglycan side chain; antibody. (A) Antibody approach. The binding of a protein to a (specific) GAG chain can be prevented by an antibody to either the (proteo-)glycan or the protein interaction partner. (A.1) The GAG-bound protein may not be targetable by the antibody raised against the soluble protein as structural rearrangements of the protein upon GAG binding as well as the change of overall/surface charge influence/destroy the antibody binding epitope. (B) Glycan- or glycan mimetic-based approach. The protein–GAG interaction can be antagonized by the addition of the synthesized protein-specific GAG sequence (or a mimetic thereof) which displaces the target protein from the natural GAG ligand. (C) Protein-based approach. The protein–GAG interaction can be antagonized by the addition of a mutant form of the target protein which exhibits higher affinity towards the natural GAG ligand and thereby displaces the wild-type target protein from the GAG chain. GAG, glycosaminoglycan.