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Sialic acid is a monosaccharide that can typically be found at the non-reducing termini of glycoconjugates, such as N- and O-glycans or glycolipids. Among all monosaccharides, sialic acid has some unique chemical characteristics. It has a 9 C-atom backbone, a carboxylic group in the C-1 position, that is deprotonated and thereby negatively charged under physiological conditions, and an amino function in the C-5 position. Although over 50 naturally occurring variants of sialic acid have been characterized to date1, the predominant form of sialic acid found in humans is N-acetylneuraminic acid (Neu5Ac). Other mammals also express higher amounts of N-glycolylneuraminic acid (Neu5Gc)2,3.
Due to its exposed position in glycoconjugates, sialic acid is involved in a plethora of receptor-ligand interactions, e.g., the hemagglutinin dependent binding of the influenza virus to host cells4. A sialic acid epitope with important biological functions, especially during embryogenesis and in the nervous system, is polysialic acid. Polysialic acid is a polymer of up to 200 alpha 2,8-linked sialic acids. The major protein carrier of polysialic acid is the neural cell adhesion molecule (NCAM). Polysialic acid expression modulates the adhesive property of NCAM in that polysialic acid expression decreases the adhesion and increases plasticity with the nervous system5.
Changes in the expression of (poly)sialic acid will ultimately affect a multitude of different biological interactions. This can be used to study known sialic acid dependent processes on a molecular level, to uncover novel glycoconjugate interactions, or explore possible therapeutic approaches. There are different methods available by which the expression of sialic acid on the cell surface can be modulated, for example treatment with sialic acid specific glycosidases (sialidases), inhibition of enzymes involved in the sialic acid biosynthesis6,7,8, or knocking down or changing the expression of the key enzyme of sialic acid biosynthesis9.
Another versatile method to modulate sialic acid expression is MGE (also known as metabolic oligosaccharide engineering, MOE). Herein, cells, tissues, or even animals are treated with non-natural derivatives of ManNAc that bear C2-amino modifications. Being precursor molecules for sialic acid, after cellular uptake, these ManNAc analogs are unidirectional metabolized to non-natural sialic acids and can be expressed on sialylated glycoconjugates. Cells treated with ManNAc derivatives carrying aliphatic C2-modifications, such as ManNProp or ManNBut, do incorporate N-propionylneuraminic acid (Neu5Prop) or N-butanoylneuraminic acid (Neu5But) in their glycoconjugates10,11. By using functional groups introduced to the C2-position of ManNAc, the occurring non-natural sialic acids can be coupled, e.g., via the Staudinger ligation or the azide alkine cycloaddition, with fluorescent dyes and therefore visualized on the cell surface12.
The expression of these non-natural sialic acids has intriguing effects on many biological processes, including pathogen infections, the adhesion and migration of tumor cells, general cell adhesion, as well as vascularization and differentiation (for review see: Wratil et al.13). Interestingly, MGE with N-acyl modified mannosamines can also be used to interfere with the expression of polysialic acid. Polysialic acid is generated by two different polysialyltransferases (ST8SiaII and ST8SiaIV). It has been demonstrated, that polysialyltransferase ST8SiaII is inhibited by unnatural sialic acid precursors, such as ManNProp or ManNBut14,15. In addition, it has been demonstrated in human neuroblastoma cells that ManNProp or ManNBut application also reduces sialylation in total15.
MGE with N-acyl modified mannosamines is an easy to apply method that has been successfully used, not only in mammalian and bacteria cell culture but also in entire animals of different species, such as Caenorhabditis elegans16, zebrafish17, or mice18,19,20,21. Especially ManNAc derivatives bearing aliphatic modifications, including ManNProp and ManNBut, are negligibly cytotoxic, even at millimolar concentrations in cell culture medium or blood plasma. Furthermore, they are relatively easy to synthesize.
Here, we provide details on how to use MGE with N-acetyl modified mannosamines. First, the chemical synthesis of two of the most widely used ManNAc derivatives in this field, ManNProp and ManNBut, is explained. Next, we show how MGE can be applied in an in vivo experiment. As an example, the neuroblastoma cell line Kelly was chosen to demonstrate decreased expression of the polysialic epitope by Western blot after treatment with the ManNAc derivatives. The non-natural sialic acids on the cell surface were quantified by HPLC and further analyzed via mass spectrometry.