N-glycosylation engineering of biopharmaceutical expression systems.

N-glycosylation, the enzymatic coupling of oligosaccharides to specific asparagine residues of nascent polypeptide chains, is one of the most widespread post-translational modifications. Following transfer of an N-glycan precursor in the ER, this structure is further modified by a number of glycosidases and glyco-syltransferases in the ER and the Golgi complex. The processing reactions occurring in the ER are highly conserved between lower and higher eukaryotes. In contrast, the reactions that take place in the Golgi complex are species- and cell type-specific. Due to its non-template driven nature, glycoproteins typically occur as a mixture of glycoforms. Since N-glycans influence circulation half-life, tissue distribution, and biological activity each glycoform has its own pharmacokinetic, pharmacodynamic and efficacy profile. Moreover, modification of glycoproteins with non-human oligosaccharides can result in undesired immunogenicity. Therefore, engineering of the N-glycosylation pathway of most currently used heterologous protein expression systems (bacteria, mammalian cells, insect cells, yeasts and plants) is actively pursued by several academic and industrial laboratories. These research efforts are in the first place directed at humanizing the N-glycosylation pathway and eliminating immunogenic glycotopes. Moreover, one wants to establish new structure-function relationships of different glycoforms, which helps to decreasing the complexity of the N-glycan repertoire towards one defined N-glycan structure. In this review, we discuss the most important recent milestones in the glycoengineering field.