Glycoproteins from insect cells: Sialylated or not?

Our growing comprehension of the biological roles of glycan moieties has cr eated a clear need for expression systems that can produce mammalian-type g lycoproteins. In turn, this has intensified interest in understanding the p rotein glycosylation pathways of the heterologous hosts that are commonly u sed for recombinant glycoprotein expression. Among these, insect cells are the most widely used and, particularly in their role as hosts for baculovir us expression vectors, provide a powerful tool for biotechnology. Various s tudies of the glycosylation patterns of endogenous and recombinant glycopro teins produced by insect cells have revealed a large variety of O- and N-li nked glycan structures and have established that the major processed O- and N-glycan species found on these glycoproteins are (Gal beta1,3)GalNAc-O-Se r/Thr and Man3(Fuc)GlcNAc2-N-Asn, respectively. However, the ability or ina bility of insect cells to synthesize and compartmentalize sialic acids and to produce sialylated glycans remains controversial. This is an important i ssue because terminal sialic acid residues play diverse biological roles in many glycoconjugates. While most work indicates that insect cell-derived g lycoproteins are not sialylated, some well-controlled studies suggest that sialylation can occur. In evaluating this work, it is important to recogniz e that oligosaccharide structural determination is tedious work, due to the infinite diversity of this class of compounds. Furthermore, there is no un iversal method of glycan analysis; rather, various strategies and technique s can be used, which provide glycobiologists with relatively more or less p recise and reliable results. Therefore, it is important to consider the met hodology used to assess glycan structures when evaluating these studies. Th e purpose of this review is to survey the studies that have contributed to our current view of glycoprotein sialylation in insect cell systems, accord ing to the methods used. Possible reasons for the disagreement on this topi c in the literature, which include the diverse origins of biological materi al and experimental artifacts, will be discussed. In the final analysis, it appears that if insect cells have the genetic potential to perform sialyla tion of glycoproteins, this is a highly specialized function that probably occurs rarely. Thus, the production of sialylated recombinant glycoproteins in the baculovirus-insect cell system will require metabolic engineering e fforts to extend the native protein glycosylation pathways of insect cells.