Recombinant therapeutic proteins produced in plants: towards engineering of human-type O- and N-glycosylation

Background and Purpose: Recombinant DNA technology has allowed expression of different heterologous proteins in many host systems, ranging from prokaryotic to eukaryotic organisms. Therapeutic properties of recombinant proteins are very often affected by the composition and heterogeneity of their glycans. Conventional expression systems for recombinant pharmaceuticals typically do not address this problem and result with products that contain a mixture of glycoforms that are neither identical to human glycans nor optimized for enhanced biological activity. Over the last decade plants have been developed as production platforms for recombinant proteins with pharmaceutical or industrial applications. Namely, plant expression systems contain very small differences in the post-translational modifications, mainly glycosylation, which can partly be overpowered by glycoengineering, whose goal is production of recombinant proteins with highly homogenous glycosylation that closely resembles the native system. This review attempts to present current accomplishments in the production of plant-derived glycoconjugates with humanized N-and O-glycans. Materials and Methods: Main goal of N-glycoengineering is to reduce or eliminate plant-specific N-glycans, and at the same time to introduce mammalian-specific N-glycans through the several approaches. The easiest way is to change intracellular targeting of plant-made recombinant proteins and to ensure their retention in the ER; next approach is to eliminate the addition of plant-specific glycans; while the final step is engineering the plant glycosylation pathway to introduce mammalian glycotransferases into plants with generation of biantennary and multi-antennary structures on complex N-glycans. Due to significant differences in O-glycosylation between humans and plants, different approaches to engineering of O-glycosylation have been taken. Besides having their typical O-glycoslyation on Hyp-residues, plants in general miss the machinery for production of mammalian-type O-glycosylation. Attempts have been made to mimic mammalian O-glycosylation in plants, specifically the mucin-type addition of GalNAc residues. Result: Efficient generation of bisected tetraantennary complex N-glycans without typical plant glycoepitopes on human erythropoietin (hEPO) and human transferrin (hTF) was obtained in Nicotiana benthamiana plants, thus demonstrating generation of recombinant proteins with human- type N-glycosylation at great uniformity. As for the O-glycosylation, attempts to produce mucin-type O-GalNAc and disialylated core 1 O-linked glycan structures on hEPO in N. benthamiana transgenic plants proved to be successful. Moreover, although small amounts of Hyp residues were found on recombinant EPO, no plant-specific O-glycans were detected, which demonstrates that plants are eligible candidates for production of recombinant therapeutics with fully humanized O- and N-glycans. Conclusion: Plants and methods of plant molecular farming offer a powerful expression platform for the production of a variety of recombinant proteins, which show similar, or even higher, biological activity then protein or native homologs in cultured mammalian cells currently used for largescale production.