Biosynthesis and Functions of KDNylated glycoproteins

Sialic acids are frequently observed as terminal modifications of polysaccharides linked to proteins. By their location and ubiquitous distribution, these sugars can influence a wide variety of physiological and pathological processes. Among the large variety of sialic acids, a seldom-studied sugar residue is the deaminated sialic acid called 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN). Unique characteristics of KDN carrying polysaccharides are its resistance to cleavage by specific human polysaccharide degrading enzymes and the presence of KDN as free sugar in certain tumor microenvironments and in end stage renal diseases in humans. There is a massive information gap on the functionality of KDN carrying polysaccharides. The main reasons are: (i) their low abundance in nature and (ii) the interference with other biosynthetic pathways. Hence, it is challenging to obtain homogeneous polysaccharides carrying KDN monosaccharides to study the functional and biochemical characteristics. There is a great interest in the engineering of glycosylation pathways to produce recombinant glycoproteins with defined glycosylation for functional studies or therapeutic applications. Compared to mammalian cells, plant-based expression systems display less of these specific polysaccharide modifications on proteins which allows higher control of the glycosylation process. Importantly, due to the abundance of precursor sugars in plants, plants provide the flexibility to introduce mammalian-like polysaccharide modifications with high efficiency by expression of the respective mammalian glycosyltransferases. To engineer the in vivo synthesis of KDN and its activated form selected biosynthetic enzymes will be transiently co-expressed in Nicotiana benthamiana plants together with recombinant proteins. The recombinant proteins will be purified and the engineered glycosylation will be analysed by mass spectrometry. Homogenously modified recombinant glycoproteins will be biochemically characterized and the effect of KDN on serum half-life, immunogenicity and receptor binding will be studied to obtain novel insights on the biological function of KDN carrying proteins.