Expression of rat beta(1,4)-N-acetylglucosaminyltransferase III in Nicotiana tabacum remodels the plant-specific N-glycosylation

Glycoengineering tobacco plants to stably express recombinant human erythropoietin with different N-glycan profiles

Plant-based expression system has many potential advantages to produce biopharmaceuticals, but plants cannot be directly used to express human glycoproteins because of their differences in glycosylation abilities from mammals. To exploit plant-based expression system for producing recombinant human erythropoietin (rhuEPO), we glycoengineered tobacco plants by stably introducing seven to eight mammalian genes including a target human EPO into tobacco in order to generate capacities for β1,4-galactosylation, bisecting N-acetylglucosamine (GlcNAc) and sialylation. Wild type human β1,4-galactosyltransferase gene (GalT) or a chimeric GalT gene (ST/GalT) was co-expressed to produce rhuEPO bearing β1,4-galactose-extended N-glycan chains as well as compare their β1,4-galactosylation efficiencies. Five mammalian genes encoding enzymes/transporter for sialic acid biosynthesis, transport and transfer were co-expressed to build sialylation capacity in plants. The human MGAT3 was co-expressed to produce N-glycan chains with bisecting GlcNAc. Our results demonstrated that the above transgenes were incorporated into tobacco genome and transcribed. ST/GalT was found to be more efficient than GalT for β1,4-galactosylation. Furthermore, co-expressing MGAT3 generated N-glycans likely bearing bisected GlcNAc. However, our current efforts did not result in generating sialylation capacity. Created transgenic plants expressing EPO and ST/GalT could be used to produce rhuEPO with high proportion of β1,4-galactose-extended N-glycan chains for tissue protective purposes.

Human protein paucimannosylation: cues from the eukaryotic kingdoms

Paucimannosidic proteins (PMPs) are bioactive glycoproteins carrying truncated α- or β-mannosyl-terminating asparagine (N)-linked glycans widely reported across the eukaryotic domain. Our understanding of human PMPs remains limited, despite findings documenting their existence and association with human disease glycobiology. This review comprehensively surveys the structures, biosynthetic routes and functions of PMPs across the eukaryotic kingdoms with the aim of synthesising an improved understanding on the role of protein paucimannosylation in human health and diseases. Convincing biochemical, glycoanalytical and biological data detail a vast structural heterogeneity and fascinating tissue- and subcellular-specific expression of PMPs within invertebrates and plants, often comprising multi-α1,3/6-fucosylation and β1,2-xylosylation amongst other glycan modifications and non-glycan substitutions e.g. O-methylation. Vertebrates and protists express less-heterogeneous PMPs typically only comprising variable core fucosylation of bi- and trimannosylchitobiose core glycans. In particular, the Manα1,6Manβ1,4GlcNAc(α1,6Fuc)β1,4GlcNAcβAsn glycan (M2F) decorates various human neutrophil proteins reportedly displaying bioactivity and structural integrity demonstrating that they are not degradation products. Less-truncated paucimannosidic glycans (e.g. M3F) are characteristic glycosylation features of proteins expressed by human cancer and stem cells. Concertedly, these observations suggest the involvement of human PMPs in processes related to innate immunity, tumorigenesis and cellular differentiation. The absence of human PMPs in diverse bodily fluids studied under many (patho)physiological conditions suggests extravascular residence and points to localised functions of PMPs in peripheral tissues. Absence of PMPs in Fungi indicates that paucimannosylation is common, but not universally conserved, in eukaryotes. Relative to human PMPs, the expression of PMPs in plants, invertebrates and protists is more tissue-wide and constitutive yet, similar to their human counterparts, PMP expression remains regulated by the physiology of the producing organism and PMPs evidently serve essential functions in development, cell-cell communication and host-pathogen/symbiont interactions. In most PMP-producing organisms, including humans, the N-acetyl-β-hexosaminidase isoenzymes and linkage-specific α-mannosidases are glycoside hydrolases critical for generating PMPs via N-acetylglucosaminyltransferase I (GnT-I)-dependent and GnT-I-independent truncation pathways. However, the identity and structure of many species-specific PMPs in eukaryotes, their biosynthetic routes, strong tissue- and development-specific expression, and diverse functions are still elusive. Deep exploration of these PMP features involving, for example, the characterisation of endogenous PMP-recognising lectins across a variety of healthy and N-acetyl-β-hexosaminidase-deficient human tissue types and identification of microbial adhesins reactive to human PMPs, are amongst the many tasks required for enhanced insight into the glycobiology of human PMPs. In conclusion, the literature supports the notion that PMPs are significant, yet still heavily under-studied biomolecules in human glycobiology that serve essential functions and create structural heterogeneity not dissimilar to other human N-glycoprotein types. Human PMPs should therefore be recognised as bioactive glycoproteins that are distinctly different from the canonical N-glycoprotein classes and which warrant a more dedicated focus in glycobiological research.

Xylosylation of proteins by expression of human xylosyltransferase 2 in plants

Through the years, the post-translational modification of plant-made recombinant proteins has been a considerable problem. Protein glycosylation is arguably the most important post-translational modification; thus, for the humanization of protein glycosylation in plants, the introduction, repression, and knockout of many glycosylation-related genes has been carried out. In addition, plants lack mammalian-type protein O-glycosylation pathways; thus, for the synthesis of mammalian O-glycans in plants, the construction of these pathways is necessary. In this study, we successfully xylosylated the recombinant human proteoglycan core protein, serglycin, by transient expression of human xylosyltransferase 2 in Nicotiana benthamiana plants. When human serglycin was co-expressed with human xylosyltransferase 2 in plants, multiple serine residues of eight xylosylation candidates were xylosylated. From the results of carbohydrate assays for total soluble proteins, some endogenous plant proteins also appeared to be xylosylated, likely through the actions of xylosyltransferase 2. The xylosylation of core proteins is the initial step of the glycosaminoglycan part of the synthesis of proteoglycans. In the future, these novel findings may lead to whole mammalian proteoglycan synthesis in plants.

Lack of the α1,3-Fucosyltransferase Gene (Osfuct) Affects Anther Development and Pollen Viability in Rice

N-linked glycosylation is one of the key post-translational modifications. α1,3-Fucosyltransferase (OsFucT) is responsible for transferring α1,3-linked fucose residues to the glycoprotein N-glycan in plants. We characterized an Osfuct mutant that displayed pleiotropic developmental defects, such as impaired anther and pollen development, diminished growth, shorter plant height, fewer tillers, and shorter panicle length and internodes under field conditions. In addition, the anthers were curved, the pollen grains were shriveled, and pollen viability and pollen number per anther decreased dramatically in the mutant. Matrix-assisted laser desorption/ionization time-of-flight analyses of the N-glycans revealed that α1,3-fucose was lacking in the N-glycan structure of the mutant. Mutant complementation revealed that the phenotype was caused by loss of Osfuct function. Transcriptome profiling also showed that several genes essential for plant developmental processes were significantly altered in the mutant, including protein kinases, transcription factors, genes involved in metabolism, genes related to protein synthesis, and hypothetical proteins. Moreover, the mutant exhibited sensitivity to an increased concentration of salt. This study facilitates a further understanding of the function of genes mediating N-glycan modification and anther and pollen development in rice.

Glyco-Engineering of Plant-Based Expression Systems

Most secreted proteins in eukaryotes are glycosylated, and after a number of common biosynthesis steps the glycan structures mature in a species-dependent manner. Therefore, human therapeutic proteins produced in plants often carry plant-like rather than human-like glycans, which can affect protein stability, biological function, and immunogenicity. The glyco-engineering of plant-based expression systems began as a strategy to eliminate plant-like glycans and produce human proteins with authentic or at least compatible glycan structures. The precise replication of human glycans is challenging, owing to the absence of a pathway in plants for the synthesis of sialylated proteins and the necessary precursors, but this can now be achieved by the coordinated expression of multiple human enzymes. Although the research community has focused on the removal of plant glycans and their replacement with human counterparts, the presence of plant glycans on proteins can also provide benefits, such as boosting the immunogenicity of some vaccines, facilitating the interaction between therapeutic proteins and their receptors, and increasing the efficacy of antibody effector functions. Graphical Abstract Typical structures of native mammalian and plant glycans with symbols indicating sugar residues identified by their short form and single-letter codes. Both glycans contain fucose, albeit with different linkages.

The next generation of antimicrobial peptides (AMPs) as molecular therapeutic tools for the treatment of diseases with social and economic impacts

Anti-infective drugs have had a key role in the contemporary world, contributing to dramatically decrease mortality rates caused by infectious diseases worldwide. Antimicrobial peptides (AMPs) are multifunctional effectors of the innate immune system of mucosal surfaces and present antimicrobial activity against a range of pathogenic viruses, bacteria, and fungi. However, the discovery and development of new antibacterial drugs is a crucial step to overcome the great challenge posed by the emergence of antibiotic resistance. In this review, we outline recent advances in the development of novel AMPs with improved antimicrobial activities that were achieved through characteristic structural design. In addition, we describe recent progress made to overcome some of the major limitations that have hindered peptide biosynthesis.

Expression of α-1,6-fucosyltransferase (FUT8) in rice grain and immunogenicity evaluation of plant-specific glycans

Rice seed is a cost-effective bioreactor for the large-scale production of pharmaceuticals. However, convincing evidence of the immunogenicity of plant-specific glycans is still limited although plant-specific glycans are considered potential allergic antigens. In the present study, we found that the α-1,3-fucose content of the glycoprotein produced from rice seed was much lower than that in leaf, and conversely, a higher β-1,2-xylose content was detected in seed than that in leaf. We detected the α-1,6-fucose content in the glutelin and recombinant human α1-antitrypsin (OsrAAT). The further results in a line containing AAT and FUT8 genes indicated that the α-1,6-fucose content of modified glycosylated recombinant α1-antitrypsin (mgOsrAAT) was 38.4%, while glutelin was only 6.8%. Interestingly, the α-1,3-fucose content of mgOsrAAT was significantly reduced by 59.8% compared with that of OsrAAT. Furthermore, we assessed the immunogenicity of OsrAAT, mgOsrAAT and human α1-antitrypsin (hAAT) using an animal system. The PCA results indicated no significant differences in the IgG, IgM and IgE titers among OsrAAT, mgOsrAAT and hAAT. Further studies revealed that those antibodies were mainly from α-1,3-fucose, but not from β-1,2-xylose, indicating that α-1,3-fucose was the major immunogenic resource. Our results demonstrated that α-1,3-fucose contents in seed proteins was much less than that of leaf, and could not be a plant-specific glycan because it also exists in human proteins.

Transient Expression of Mammalian Genes in N. benthamiana to Modulate N-Glycosylation

Nicotiana benthamiana has shown great success as a platform for the production of recombinant proteins. Here, we describe methods to transiently express high levels of recombinant proteins and simultaneously modulate their glycosylation pattern toward human-like structures. The method aims to generate recombinant proteins with a targeted largely homogeneous glycosylation profile for structure-function studies.

Characterization of plants expressing the human β1,4-galactosyltrasferase gene

Modification of the plant N-glycosylation pathway towards human type structures is an important strategy to implement plants as expression systems for therapeutic proteins. Nevertheless, relatively little is known about the overall impact of non-plant glycosylation enzymes in stable transformed plants. Here, we analyzed transgenic lines (Nicotiana benthamiana and Arabidopsis thaliana) that stably express a modified version of human β1,4-galactosyltransferase ((ST)GalT). While some transgenic plants grew normally, other lines exhibited a severe phenotype associated with stunted growth and developmental retardation. The severity of the phenotype correlated with both increased (ST)GalT mRNA and protein levels but no differences were observed between N-glycosylation profiles of plants with and without the phenotype. In contrast to non-transgenic plants, all (ST)GalT expressing plants synthesized significant amounts of incompletely processed (largely depleted of core fucose) N-glycans with up to 40% terminally galactosylated structures. While transgenic plants showed no differences in nucleotide sugar composition and cell wall monosaccharide content, alterations in the reactivity of cell wall carbohydrate epitopes associated with arabinogalactan-proteins and pectic homogalacturonan were detected in (ST)GalT expressing plants. Notably, plants with phenotypic alterations showed increased levels of hydrogen peroxide, most probably a consequence of hypersensitive reactions. Our data demonstrate that unfavorable phenotypical modifications may occur upon stable in planta expression of non-native glycosyltransferases. Such important issues need to be taken into consideration in respect to stable glycan engineering in plants.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

N-Glyco-Engineering in Plants: Update on Strategies and Major Achievements

Plants are being developed as a cost-effective production system for biopharmaceuticals in large quantities. Although plants properly fold and assemble complex proteins from human origin, one issue that needs to be addressed is their glycan structure. In the past years we have been witnessing outstanding results in targeted manipulation of the plant N-glycosylation pathway allowing recombinant proteins to be produced with human-type oligosaccharides at large homogeneity. This opens new possibility in manufacturing next-generation biopharmaceuticals.This review presents a variety of technologies and strategies that are being employed to engineer the plant N-glycosylation, thus pointing to the enormous potential of plants being used as a novel production system with unique features and possibilities.

Im"plant"ing of Mammalian Glycosyltransferase Gene into Plant Suspension-Cultured Cells Using Agrobacterium-Mediated Transformation

Enzymatic activity assay of exogenous glycosyltransferase (GT) and glycosylhydrolase (GH) expressed in plants is an important analysis for determination of the expression of the gene of interest. However, generations and establishment of in planta transgenic lines are time-consuming. Furthermore, the expression levels and the activities of the exogenous GTs and GHs are quite low and weak, the radiolabeled donor substrate had to be used to analyze the enzymatic activity. Here, we describe a protocol for the generation of transgenic plants using suspension-cultured cells and a high sensitive assay for GT, especially β1,4-galactosyltransferase, using microsomal fraction from plant cells and fluorescent-labeled sugar chains as an acceptor substrate. This method enables less-time-consuming preparation of stable transgenic plants, non-radiolabeled, high-throughput detail analysis which includes mass spectrometric analysis and exo-glycosidase digestions.

On the way to commercializing plant cell culture platform for biopharmaceuticals: present status and prospect

Plant cell culture is emerging as an alternative bioproduction system for recombinant pharmaceuticals. Growing plant cells in vitro under controlled environmental conditions allows for precise control over cell growth and protein production, batch-to-batch product consistency and a production process aligned with current good manufacturing practices. With the recent US FDA approval and commercialization of the world's first plant cell-based recombinant pharmaceutical for human use, β-glucocerebrosidase for treatment of Gaucher's disease, a new era has come in which plant cell culture shows high potential to displace some established platform technologies in niche markets. This review updates the progress in plant cell culture processing technology, highlights recent commercial successes and discusses the challenges that must be overcome to make this platform commercially viable.

Plant glyco-biotechnology on the way to synthetic biology

Plants are increasingly being used for the production of recombinant proteins. One reason is that plants are highly amenable to glycan engineering processes and allow the production of therapeutic proteins with increased efficacies due to optimized glycosylation profiles. Removal and insertion of glycosylation reactions by knock-out/knock-down approaches and introduction of glycosylation enzymes have paved the way for the humanization of the plant glycosylation pathway. The insertion of heterologous enzymes at exactly the right stage of the existing glycosylation pathway has turned out to be of utmost importance. To enable such precise targeting chimeric enzymes have been constructed. In this short review we will exemplify the importance of correct targeting of glycosyltransferases, we will give an overview of the targeting mechanism of glycosyltransferases, describe chimeric enzymes used in plant N-glycosylation engineering and illustrate how plant glycoengineering builds on the tools offered by synthetic biology to construct such chimeric enzymes.

Biological significance of complex N-glycans in plants and their impact on plant physiology

Asparagine (N)-linked protein glycosylation is a ubiquitous co- and post-translational modification which can alter the biological function of proteins and consequently affects the development, growth, and physiology of organisms. Despite an increasing knowledge of N-glycan biosynthesis and processing, we still understand very little about the biological function of individual N-glycan structures in plants. In particular, the N-glycan-processing steps mediated by Golgi-resident enzymes create a structurally diverse set of protein-linked carbohydrate structures. Some of these complex N-glycan modifications like the presence of β1,2-xylose, core α1,3-fucose or the Lewis a-epitope are characteristic for plants and are evolutionary highly conserved. In mammals, complex N-glycans are involved in different cellular processes including molecular recognition and signaling events. In contrast, the complex N-glycan function is still largely unknown in plants. Here, in this short review, I focus on important recent developments and discuss their implications for future research in plant glycobiology and plant biotechnology.

Glyco-engineering in plants to produce human-like N-glycan structures

It is now possible to produce complex human proteins, largely correctly folded and N-glycosylated, in plants. Much effort has been invested in engineering expression technologies to develop products with superior characteristics. The results have begun to show success in controlling important posttranslational modifications such as N-glycosylation. With the emerging data increasingly indicating the significance of proper N-glycosylation for the efficacy of a drug, glyco-engineering has become an important issue not only for academia but also for the biopharmaceutical industry. Plants have demonstrated a high degree of tolerance to changes in the N-glycosylation pathway, allowing recombinant proteins to be modified into human-like structures in a specific and controlled manner. Frequently the results are a largely homogeneously glycosylated product, currently unrivalled by that of any other expression platforms. This review provides a comprehensive analysis of recent advances in plant N-glyco-engineering in the context of the expression of therapeutically relevant proteins, highlighting both the challenges and successes in the application of such powerful technologies.

A protease activity-depleted environment for heterologous proteins migrating towards the leaf cell apoplast

Recombinant proteins face major constraints along the plant cell secretory pathway, including proteolytic processing compromising their structural integrity. Here, we demonstrate the potential of protease inhibitors as in situ stabilizing agents for recombinant proteins migrating towards the leaf apoplast. Genomic data for Arabidopsis, rice and Nicotiana spp. were assessed to determine the relative incidence of protease families in the cell secretory pathway. Transient expression assays with the model platform Nicotiana benthamiana were then performed to test the efficiency of protease inhibitors in stabilizing proteins targeted to the apoplast. Current genomic data suggest the occurrence of proteases from several families along the secretory pathway, including A1 and A22 Asp proteases; C1A and C13 Cys proteases; and S1, S8 and S10 Ser proteases. In vitro protease assays confirmed the presence of various proteases in N. benthamiana leaves, notably pointing to the deposition of A1- and S1-type activities preferentially in the apoplast. Accordingly, transient expression and secretion of the A1/S1 protease inhibitor, tomato cathepsin D inhibitor (SlCDI), negatively altered A1 and S1 protease activities in this cell compartment, while increasing the leaf apoplast protein content by ∼45% and improving the accumulation of a murine diagnostic antibody, C5-1, co-secreted in the apoplast. SlCYS9, an inhibitor of C1A and C13 Cys proteases, had no impact on the apoplast proteases and protein content, but stabilized C5-1 in planta, presumably upstream in the secretory pathway. These data confirm, overall, the potential of protease inhibitors for the in situ protection of recombinant proteins along the plant cell secretory pathway.

Production of recombinant human granulocyte macrophage-colony stimulating factor in rice cell suspension culture with a human-like N-glycan structure

The rice α-amylase 3D promoter system, which is activated under sucrose-starved conditions, has emerged as a useful system for producing recombinant proteins. However, using rice as the production system for therapeutic proteins requires modifications of the N-glycosylation pattern because of the potential immunogenicity of plant-specific sugar residues. In this study, glyco-engineered rice were generated as a production host for therapeutic glycoproteins, using RNA interference (RNAi) technology to down-regulate the endogenous α-1,3-fucosyltransferase (α-1,3-FucT) and β-1,2-xylosyltransferase (β-1,2-XylT) genes. N-linked glycans from the RNAi lines were identified, and their structures were compared with those isolated from a wild-type cell suspension. The inverted-repeat chimeric RNA silencing construct of α-1,3-fucosyltransferase and β-1,2-xylosyltransferase (Δ3FT/XT)-9 glyco-engineered line with significantly reduced core α-1,3-fucosylated and/or β-1,2-xylosylated glycan structures was established. Moreover, levels of plant-specific α-1,3-fucose and/or β-1,2-xylose residues incorporated into recombinant human granulocyte/macrophage colony-stimulating factor (hGM-CSF) produced from the N44 + Δ3FT/XT-4 glyco-engineered line co-expressing ihpRNA of Δ3FT/XT and hGM-CSF were significantly decreased compared with those in the previously reported N44-08 transgenic line expressing hGM-CSF. None of the glyco-engineered lines differed from the wild type with respect to cell division, proliferation or ability to secrete proteins into the culture medium.

N-glycosylation engineering of plants for the biosynthesis of glycoproteins with bisected and branched complex N-glycans

Glycoengineering is increasingly being recognized as a powerful tool to generate recombinant glycoproteins with a customized N-glycosylation pattern. Here, we demonstrate the modulation of the plant glycosylation pathway toward the formation of human-type bisected and branched complex N-glycans. Glycoengineered Nicotiana benthamiana lacking plant-specific N-glycosylation (i.e. β1,2-xylose and core α1,3-fucose) was used to transiently express human erythropoietin (hEPO) and human transferrin (hTF) together with modified versions of human β1,4-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnTIII), α1,3-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnTIV) and α1,6-mannosyl-β1,6-N-acetylglucosaminyltransferase (GnTV). hEPO was expressed as a fusion to the IgG-Fc domain (EPO-Fc) and purified via protein A affinity chromatography. Recombinant hTF was isolated from the intracellular fluid of infiltrated plant leaves. Mass spectrometry-based N-glycan analysis of hEPO and hTF revealed the quantitative formation of bisected (GnGnbi) and tri- as well as tetraantennary complex N-glycans (Gn[GnGn], [GnGn]Gn and [GnGn][GnGn]). Co-expression of GnTIII together with GnTIV and GnTV resulted in the efficient generation of bisected tetraantennary complex N-glycans. Our results show the generation of recombinant proteins with human-type N-glycosylation at great uniformity. The strategy described here provides a robust and straightforward method for producing mammalian-type N-linked glycans of defined structures on recombinant glycoproteins, which can advance glycoprotein research and accelerate the development of protein-based therapeutics.

Deletion of fucose residues in plant N-glycans by repression of the GDP-mannose 4,6-dehydratase gene using virus-induced gene silencing and RNA interference

Production of pharmaceutical glycoproteins in plants has many advantages in terms of safety and reduced costs. However, plant-produced glycoproteins have N-glycans with plant-specific sugar residues (core β-1,2-xylose and α-1,3-fucose) and a Lewis a (Le(a) ) epitope, i.e., Galβ(1-3)[Fucα(1-4)]GlcNAc. Because these sugar residues and glycan structures seemed to be immunogenic, several attempts have been made to delete them by repressing their respective glycosyltransferase genes. However, until date, such deletions have not been successful in completely eliminating the fucose residues. In this study, we simultaneously reduced the plant-specific core α-1,3-fucose and α-1,4-fucose residues in the Le(a) epitopes by repressing the Guanosine 5'-diphosphate (GDP)-D-mannose 4,6-dehydratase (GMD) gene, which is associated with GDP-L-fucose biosynthesis, in Nicotiana benthamiana plants. Repression of GMD was achieved using virus-induced gene silencing (VIGS) and RNA interference (RNAi). The proportion of fucose-free N-glycans found in total soluble protein from GMD gene-repressed plants increased by 80% and 95% following VIGS and RNAi, respectively, compared to wild-type plants. A small amount of putative galactose substitution in N-glycans from the NbGMD gene-repressed plants was observed, similar to what has been previously reported GMD-knockout Arabidopsis mutant. On the other hand, the recombinant mouse granulocyte-macrophage colony-stimulating factor (GM-CSF) with fucose-deleted N-glycans was successfully produced in NbGMD-RNAi transgenic N. benthamiana plants. Thus, repression of the GMD gene is thus very useful for deleting immunogenic total fucose residues and facilitating the production of pharmaceutical glycoproteins in plants.

Beta-N-acetylhexosaminidases HEXO1 and HEXO3 are responsible for the formation of paucimannosidic N-glycans in Arabidopsis thaliana

Most plant glycoproteins contain substantial amounts of paucimannosidic N-glycans instead of their direct biosynthetic precursors, complex N-glycans with terminal N-acetylglucosamine residues. We now demonstrate that two β-N-acetylhexosaminidases (HEXO1 and HEXO3) residing in different subcellular compartments jointly account for the formation of paucimannosidic N-glycans in Arabidopsis thaliana. Total N-glycan analysis of hexo knock-out plants revealed that HEXO1 and HEXO3 contribute equally to the production of paucimannosidic N-glycans in roots, whereas N-glycan processing in leaves depends more heavily on HEXO3 than on HEXO1. Because hexo1 hexo3 double mutants do not display any obvious phenotype even upon exposure to different forms of abiotic or biotic stress, it should be feasible to improve the quality of glycoprotein therapeutics produced in plants by down-regulation of endogenous β-N-acetylhexosaminidase activities.

Review article: commercialization of whole-plant systems for biomanufacturing of protein products: evolution and prospects

Technology for enabling plants to biomanufacture nonnative proteins in commercially significant quantities has been available for just over 20 years. During that time, the agricultural world has witnessed rapid commercialization and widespread adoption of transgenic crops enhanced for agronomic performance (herbicide-tolerance, insect-resistance), while plant-made pharmaceuticals (PMPs) and plant-made industrial products (PMIPs) have been limited to experimental and small-scale commercial production. This difference in the rate of commercial implementation likely reflects the very different business-development challenges associated with 'product' technologies compared with 'enabling' ('platform') technologies. However, considerable progress has been made in advancing and refining plant-based production of proteins, both technologically and in regard to identifying optimal business prospects. This review summarizes these developments, contrasting today's technologies and prospective applications with those of the industry's formative years, and suggesting how the PM(I)P industry's evolution has generated a very positive outlook for the 'plant-made' paradigm.

Plant-produced trastuzumab inhibits the growth of HER2 positive cancer cells

To study the agricultural production of biosimilar antibodies, trastuzumab (Herceptin) was expressed in Nicotiana benthamiana using the magnICON viral-based transient expression system. Immunoblot analyses of crude plant extracts revealed that trastuzumab accumulates within plants mostly in the fully assembled tetrameric form. Purification of trastuzumab from N. benthamiana was achieved using a scheme that combined ammonium sulfate precipitation with affinity chromatography. Following purification, the specificity of the plant-produced trastuzumab for the HER2 receptor was compared with Herceptin and confirmed by western immunoblot. Functional assays revealed that plant-produced trastuzumab and Herceptin have similar in vitro antiproliferative effects on breast cancer cells that overexpress HER2. Results confirm that plants may be developed as an alternative to traditional antibody expression systems for the production of therapeutic mAbs.

Reduction of N-linked xylose and fucose by expression of rat beta1,4-N-acetylglucosaminyltransferase III in tobacco BY-2 cells depends on Golgi enzyme localization domain and genetic elements used for expression

Plant-specific N-glycosylation, such as the introduction of core alpha1,3-fucose and beta1,2-xylose residues, is a major obstacle to the utilization of plant cell- or plant-derived recombinant therapeutic proteins. The beta1,4-N-acetylglucosaminyltransferase III (GnTIII) introduces a bisecting GlcNAc residue into N-glycans, which exerts a high level of substrate mediated control over subsequent modifications, for example inhibiting mammalian core fucosylation. Based on similar findings in plants, we used Nicotianatabacum BY-2 cells to study the effects of localization and expression levels of GnTIII in the remodeling of the plant N-glycosylation pathway. The N-glycans produced by the cells expressing GnTIII were partially bisected and practically devoid of the paucimannosidic type which is typical for N-glycans produced by wildtype BY-2 suspension cultured cells. The proportion of human-compatible N-glycans devoid of fucose and xylose could be increased from an average of 4% on secreted protein from wildtype cells to as high as 59% in cells expressing chimeric GnTIII, named GnTIII(A.th.) replacing its native localization domain with the cytoplasmic tail, transmembrane, and stem region of Arabidopsis thaliana mannosidase II. The changes in N-glycosylation observed were dependent on the catalytic activity of GnTIII, as the expression of catalytically inactive GnTIII mutants did not show a significant effect on N-glycosylation.

A companion protease inhibitor for the protection of cytosol-targeted recombinant proteins in plants

We reported earlier the potential of tomato cathepsin D inhibitor (SlCDI) as an in-built stabilizing agent for the protection of recombinant proteins in transgenic plant leaf crude extracts (Plant Biotechnol J.4, 359-368). Here we document the potential of SlCDI for the in situ protection of proteins in potato leaves. Total protein assays with control and SlCDI-expressing potato lines indicated a positive impact of slcdi transgene expression on leaf protein content, with a mean relative increase of 35%-40% depending on the light regime. Out of approximately 700 proteins detected on 2-D gels, only 20 exhibited a significantly altered level on a protein-specific basis, whereas most proteins were up-regulated on a leaf fresh weight basis, albeit at variable rates. Quantitative reverse trancriptase-PCR assays for rubisco activase showed similar transcript levels in leaves of test and control lines despite protein levels increased by two- to threefold in SlCDI-expressing lines. These observations, along with the unrelated biological functions assigned to MS-identified proteins up-regulated in leaves and protease assays showing slightly increased proteasome activity in protein extracts of SlCDI-expressing lines, suggest a general, proteasome-independent protein stabilizing effect of SlCDI in planta. Transient expression assays with human alpha(1)-antichymotrypsin also showed a stabilizing effect for SlCDI on heterologous proteins, leading to net levels of the human protein increased by approximately 2.5-fold in SlCDI-expressing plants. These data illustrate, overall, the potential of SlCDI as an in vivo protein-stabilizing agent in transgenic plant systems, useful to improve protein levels and recombinant protein accumulation.

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.