Engineering of a sialic acid synthesis pathway in transgenic plants by expression of bacterial Neu5Ac-synthesizing enzymes

In vivo deglycosylation of recombinant glycoproteins in tobacco BY-2 cells

Production of recombinant pharmaceutical glycoproteins has been carried out in multiple expression systems. However, N-glycosylation, which increases heterogeneity and raises safety concerns due to the presence of non-human residues, is usually not controlled. The presence and composition of N-glycans are also susceptible to affect protein stability, function and immunogenicity. To tackle these issues, we are developing glycoengineered Nicotiana tabacum Bright Yellow-2 (BY-2) cell lines through knock out and ectopic expression of genes involved in the N-glycosylation pathway. Here, we report on the generation of BY-2 cell lines producing deglycosylated proteins. To this end, endoglycosidase T was co-expressed with an immunoglobulin G or glycoprotein B of human cytomegalovirus in BY-2 cell lines producing only high mannose N-glycans. Endoglycosidase T cleaves high mannose N-glycans to generate single, asparagine-linked, N-acetylglucosamine residues. The N-glycosylation profile of the secreted antibody was determined by mass spectrometry analysis. More than 90% of the N-glycans at the conserved Asn297 site were deglycosylated. Likewise, extensive deglycosylation of glycoprotein B, which possesses 18 N-glycosylation sites, was observed. N-glycan composition of gB glycovariants was assessed by in vitro enzymatic mobility shift assay and proven to be consistent with the expected glycoforms. Comparison of IgG glycovariants by differential scanning fluorimetry revealed a significant impact of the N-glycosylation pattern on the thermal stability. Production of deglycosylated pharmaceutical proteins in BY-2 cells expands the set of glycoengineered BY-2 cell lines.

Cellular engineering of plant cells for improved therapeutic protein production

In vitro cultured plant cells, in particular the tobacco BY-2 cell, have demonstrated their potential to provide a promising bioproduction platform for therapeutic proteins by integrating the merits of whole-plant cultivation systems with those of microbial and mammalian cell cultures. Over the past three decades, substantial progress has been made in improving the plant cell culture system, resulting in a few commercial success cases, such as taliglucerase alfa (Elelyso^®), the first FDA-approved recombinant pharmaceutical protein derived from plant cells. However, compared to the major expression hosts (bacteria, yeast, and mammalian cells), plant cells are still largely underutilized, mainly due to low productivity and non-human glycosylation. Modern molecular biology tools, in particular RNAi and the latest genome editing technology CRISPR/Cas9, have been used to modulate the genome of plant cells to create new cell lines that exhibit desired "traits" for producing therapeutic proteins. This review highlights the recent advances in cellular engineering of plant cells towards improved recombinant protein production, including creating cell lines with deficient protease levels or humanized glycosylation, and considers potential development in the future.

Engineering of Plants for Efficient Production of Therapeutics

Plants are becoming useful platforms for recombinant protein production at present time. With the advancement of efficient molecular tools of genomics, proteomics, plants are now being used as a biofactory for production of different life saving therapeutics. Plant-based biofactory is an established production system with the benefits of cost-effectiveness, high scalability, rapid production, enabling post-translational modification, and being devoid of harmful pathogens contamination. This review introduces the main challenges faced by plant expression system: post-translational modifications, protein stability, biosafety concern and regulation. It also summarizes essential factors to be considered in engineering plants, including plant expression system, promoter, post-translational modification, codon optimization, and fusion tags, protein stabilization and purification, subcellular targeting, and making vaccines in an edible way. This review will be beneficial and informative to scholars and readers in the field of plant biotechnology.

Stable Protein Sialylation in Physcomitrella

Recombinantly produced proteins are indispensable tools for medical applications. Since the majority of them are glycoproteins, their N-glycosylation profiles are major determinants for their activity, structural properties and safety. For therapeutical applications, a glycosylation pattern adapted to product and treatment requirements is advantageous. Physcomitrium patens (Physcomitrella, moss) is able to perform highly homogeneous complex-type N-glycosylation. Additionally, it has been glyco-engineered to eliminate plant-specific sugar residues by knock-out of the β1,2-xylosyltransferase and α1,3-fucosyltransferase genes (Δxt/ft). Furthermore, Physcomitrella meets wide-ranging biopharmaceutical requirements such as GMP compliance, product safety, scalability and outstanding possibilities for precise genome engineering. However, all plants, in contrast to mammals, lack the capability to perform N-glycan sialylation. Since sialic acids are a common terminal modification on human N-glycans, the property to perform N-glycan sialylation is highly desired within the plant-based biopharmaceutical sector. In this study, we present the successful achievement of protein N-glycan sialylation in stably transformed Physcomitrella. The sialylation ability was achieved in a Δxt/ft moss line by stable expression of seven mammalian coding sequences combined with targeted organelle-specific localization of the encoded enzymes responsible for the generation of β1,4-galactosylated acceptor N-glycans as well as the synthesis, activation, transport and transfer of sialic acid. Production of free (Neu5Ac) and activated (CMP-Neu5Ac) sialic acid was proven. The glycosidic anchor for the attachment of terminal sialic acid was generated by the introduction of a chimeric human β1,4-galactosyltransferase gene under the simultaneous knock-out of the gene encoding the endogenous β1,3-galactosyltransferase. Functional complex-type N-glycan sialylation was confirmed via mass spectrometric analysis of a stably co-expressed recombinant human protein.

N- and O-Glycosylation Pathways in the Microalgae Polyphyletic Group

The term microalga refers to various unicellular and photosynthetic organisms representing a polyphyletic group. It gathers numerous species, which can be found in cyanobacteria (i.e., Arthrospira) as well as in distinct eukaryotic groups, such as Chlorophytes (i.e., Chlamydomonas or Chlorella) and Heterokonts (i.e., diatoms). This phylogenetic diversity results in an extraordinary variety of metabolic pathways, offering large possibilities for the production of natural compounds like pigments or lipids that can explain the ever-growing interest of industrials for these organisms since the middle of the last century. More recently, several species have received particular attention as biofactories for the production of recombinant proteins. Indeed, microalgae are easy to grow, safe and cheap making them attractive alternatives as heterologous expression systems. In this last scope of applications, the glycosylation capacity of these organisms must be considered as this post-translational modification of proteins impacts their structural and biological features. Although these mechanisms are well known in various Eukaryotes like mammals, plants or insects, only a few studies have been undertaken for the investigation of the protein glycosylation in microalgae. Recently, significant progresses have been made especially regarding protein N-glycosylation, while O-glycosylation remain poorly known. This review aims at summarizing the recent data in order to assess the state-of-the art knowledge in glycosylation processing in microalgae.

Sialoglycans and genetically engineered plants

Plants express N-glycosylation pathways and produce N-glycosylated proteins but differ from the mammalian-type proteins. Therefore attempts are made to design and engineer plant glycosylation pathways that can produce mammalian-type glycosylated moieties so that large quantities of biopharmaceuticals compatible to the human body can be produced. Most of the studies of plant expression systems for molecular farming have been conducted on Nicotiana sp. and genetic engineering and molecular biology tools have enabled the generation of glycoengineered plant for human use in the production of therapeutic recombinant proteins. We have discussed in this chapter the advances of glycoengineering in plants with special reference to the reconstruction of silaylation pathways in plants and the latest application in the production of antibody and therapeutics in plants.

Production of complex viral glycoproteins in plants as vaccine immunogens

Plant molecular farming offers a cost-effective and scalable approach to the expression of recombinant proteins which has been proposed as an alternative to conventional production platforms for developing countries. In recent years, numerous proofs of concept have established that plants can produce biologically active recombinant proteins and immunologically relevant vaccine antigens that are comparable to those made in conventional expression systems. Driving many of these advances is the remarkable plasticity of the plant proteome which enables extensive engineering of the host cell, as well as the development of improved expression vectors facilitating higher levels of protein production. To date, the only plant-derived viral glycoprotein to be tested in humans is the influenza haemagglutinin which expresses at ~50 mg/kg. However, many other viral glycoproteins that have potential as vaccine immunogens only accumulate at low levels in planta. A critical consideration for the production of many of these proteins in heterologous expression systems is the complexity of post-translational modifications, such as control of folding, glycosylation and disulphide bridging, which is required to reproduce the native glycoprotein structure. In this review, we will address potential shortcomings of plant expression systems and discuss strategies to optimally exploit the technology for the production of immunologically relevant and structurally authentic glycoproteins for use as vaccine immunogens.

Platforms for Plant-Based Protein Production

Plant molecular farming depends on a diversity of plant systems for production of useful recombinant proteins. These proteins include protein biopolymers, industrial proteins and enzymes, and therapeutic proteins. Plant production systems include microalgae, cells, hairy roots, moss, and whole plants with both stable and transient expression. Production processes involve a narrowing diversity of bioreactors for cell, hairy root, microalgae, and moss cultivation. For whole plants, both field and automated greenhouse cultivation methods are used with products expressed and produced either in leaves or seeds. Many successful expression systems now exist for a variety of different products with a list of increasingly successful commercialized products. This chapter provides an overview and examples of the current state of plant-based production systems for different types of recombinant proteins.

Transient Expression of Tetrameric Recombinant Human Butyrylcholinesterase in Nicotiana benthamiana

To optimize the expression, extraction and purification of plant-derived tetrameric recombinant human butyrylcholinesterase (prBChE), we describe the development and use of plant viral amplicon-based gene expression system; Tobacco Mosaic Virus (TMV) RNA-based overexpression vector (TRBO) to express enzymatically active FLAG-tagged plant made recombinant butyrylcholinesterase (rBChE) in Nicotiana benthamiana leaves using transient agroinfiltration. Two gene expression cassettes were designed to express the recombinant protein in either the ER or to the apoplastic compartment. Leaf homogenization was used to isolate ER-retained recombinant butyrylcholinesterase (prBChE-ER) while apoplast-targeted rBChE was isolated by either leaf homogenization (prBChE) or vacuum-extraction of apoplastic wash fluid (prBChE-AWF). rBChE from apoplast wash fluid had a higher specific activity but lower enzyme yield than leaf homogenate. To optimize the isolation and purification of total recombinant protein from leaf homogenates, an acidic extraction buffer was used. The acidic extraction buffer yielded >95% enzymatically active tetrameric rBChE as verified by Coomassie stained and native gel electrophoresis. Furthermore, when compared to human butyrylcholinesterase, the prBChE was found to be similar in terms of tetramerization and enzyme kinetics. The N-linked glycan profile of purified prBChE-ER was found to be mostly high mannose structures while the N-linked glycans on prBChE-AWF were primarily complex. The glycan profile of the prBChE leaf homogenates showed a mixture of high mannose, complex and paucimannose type N-glycans. These findings demonstrate the ability of plants to produce rBChE that is enzymatically active and whose oligomeric state is comparable to mammalian butyrylcholinesterase. The process of plant made rBChE tetramerization and strategies for improving its pharmacokinetics properties are also discussed.

Characterization of a novel N-acetylneuraminic acid lyase favoring industrial N-acetylneuraminic acid synthesis

N-Acetylneuraminic acid lyase (NAL, E.C. number 4.1.3.3) is a Class I aldolase that catalyzes the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac) from pyruvate and N-acetyl-D-mannosamine (ManNAc). Due to the equilibrium favoring Neu5Ac cleavage, the enzyme catalyzes the rate-limiting step of two biocatalytic reactions producing Neu5Ac in industry. We report the biochemical characterization of a novel NAL from a “GRAS” (General recognized as safe) strain C. glutamicum ATCC 13032 (CgNal). Compared to all previously reported NALs, CgNal exhibited the lowest k_cat/K_m value for Neu5Ac and highest k_cat/K_m values for ManNAc and pyruvate, which makes CgNal favor Neu5Ac synthesis the most. The recombinant CgNal reached the highest expression level (480 mg/L culture), and the highest reported yield of Neu5Ac was achieved (194 g/L, 0.63 M). All these unique properties make CgNal a promising biocatalyst for industrial Neu5Ac biosynthesis. Additionally, although showing the best Neu5Ac synthesis activity among the NAL family, CgNal is more related to dihydrodipicolinate synthase (DHDPS) by phylogenetic analysis. The activities of CgNal towards both NAL's and DHDPS' substrates are fairly high, which indicates CgNal a bi-functional enzyme. The sequence analysis suggests that CgNal might have adopted a unique set of residues for substrates recognition.

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.

Protein N-glycosylation in eukaryotic microalgae and its impact on the production of nuclear expressed biopharmaceuticals

Microalgae are currently used for the production of food compounds. Recently, few microalgae species have been investigated as potential biofactories for the production of biopharmaceuticals. Indeed in this context, microalgae are cheap, classified as Generally Recognized As Safe (GRAS) organisms and can be grown easily. However, problems remain to be solved before any industrial production of microalgae-made biopharmaceuticals. Among them, post-translational modifications of the proteins need to be considered. Especially, N-glycosylation acquired by the secreted recombinant proteins is of major concern since most of the biopharmaceuticals are N-glycosylated and it is well recognized that glycosylation represent one of their critical quality attribute. Therefore, the evaluation of microalgae as alternative cell factory for biopharmaceutical productions thus requires to investigate their N-glycosylation capability in order to determine to what extend it differs from their human counterpart and to determine appropriate strategies for remodeling the microalgae glycosylation into human-compatible oligosaccharides. Here, we review the secreted recombinant proteins which have been successfully produced in microalgae. We also report on recent bioinformatics and biochemical data concerning the structure of glycans N-linked to proteins from various microalgae phyla and comment the consequences on the glycan engineering strategies that may be necessary to render those microalgae-made biopharmaceuticals compatible with human therapy.

Quantitative evaluation of E1 endoglucanase recovery from tobacco leaves using the vacuum infiltration-centrifugation method

As a production platform for recombinant proteins, plant leaf tissue has many advantages, but commercialization of this technology has been hindered by high recovery and purification costs. Vacuum infiltration-centrifugation (VI-C) is a technique to obtain extracellularly-targeted products from the apoplast wash fluid (AWF). Because of its selective recovery of secreted proteins without homogenizing the whole tissue, VI-C can potentially reduce downstream production costs. Lab scale experiments were conducted to quantitatively evaluate the VI-C method and compared to homogenization techniques in terms of product purity, concentration, and other desirable characteristics. From agroinfiltrated Nicotiana benthamiana leaves, up to 81% of a truncated version of E1 endoglucanase from Acidothermus cellulolyticus was recovered with VI-C versus homogenate extraction, and average purity and concentration increases of 4.2-fold and 3.1-fold, respectively, were observed. Formulas were developed to predict recovery yields of secreted protein obtained by performing multiple rounds of VI-C on the same leaf tissue. From this, it was determined that three rounds of VI-C recovered 97% of the total active recombinant protein accessible to the VI-C procedure. The results suggest that AWF recovery is an efficient process that could reduce downstream processing steps and costs for plant-made recombinant proteins.

Lepidopteran cells, an alternative for the production of recombinant antibodies?

Monoclonal antibodies are used with great success in many different therapeutic domains. In order to satisfy the growing demand and to lower the production cost of these molecules, many alternative systems have been explored. Among them, the baculovirus/insect cells system is a good candidate. This system is very safe, given that the baculoviruses have a highly restricted host range and they are not pathogenic to vertebrates or plants. But the major asset is the speed with which it is possible to obtain very stable recombinant viruses capable of producing fully active proteins whose glycosylation pattern can be modulated to make it similar to the human one. These features could ultimately make the difference by enabling the production of antibodies with very low costs. However, efforts are still needed, in particular to increase production rates and thus make this system commercially viable for the production of these therapeutic agents.

Overproduction of recombinant proteins in plants

Recombinant protein production in microbial hosts and animal cell cultures has revolutionized the pharmaceutical and industrial enzyme industries. Plants as alternative hosts for the production of recombinant proteins are being actively pursued, taking advantage of their unique characteristics. The key to cost-efficient production in any system is the level of protein accumulation, which is inversely proportional to the cost. Levels of up to 5 g/kg biomass have been obtained in plants, making this production system competitive with microbial hosts. Increasing protein accumulation at the cellular level by varying host, germplasm, location of protein accumulation, and transformation procedure is reviewed. At the molecular level increased expression by improving transcription, translation and accumulation of the protein is critically evaluated. The greatest increases in protein accumulation will occur when various optimized parameters are more fully integrated with each other. Because of the complex nature of plants, this will take more time and effort to accomplish than has been the case for the simpler unicellular systems. However the potential for plants to become one of the major avenues for protein production appears very promising.

Synthesis of an antiviral drug precursor from chitin using a saprophyte as a whole-cell catalyst

Background

Recent incidents, such as the SARS and influenza epidemics, have highlighted the need for readily available antiviral drugs. One important precursor currently used for the production of Relenza, an antiviral product from GlaxoSmithKline, is N-acetylneuraminic acid (NeuNAc). This substance has a considerably high market price despite efforts to develop cost-reducing (biotechnological) production processes. Hypocrea jecorina (Trichoderma reesei) is a saprophyte noted for its abundant secretion of hydrolytic enzymes and its potential to degrade chitin to its monomer N-acetylglucosamine (GlcNAc). Chitin is considered the second most abundant biomass available on earth and therefore an attractive raw material.

Results

In this study, we introduced two enzymes from bacterial origin into Hypocrea, which convert GlcNAc into NeuNAc via N-acetylmannosamine. This enabled the fungus to produce NeuNAc from the cheap starting material chitin in liquid culture. Furthermore, we expressed the two recombinant enzymes as GST-fusion proteins and developed an enzyme assay for monitoring their enzymatic functionality. Finally, we demonstrated that Hypocrea does not metabolize NeuNAc and that no NeuNAc-uptake by the fungus occurs, which are important prerequisites for a potential production strategy.

Conclusions

This study is a proof of concept for the possibility to engineer in a filamentous fungus a bacterial enzyme cascade, which is fully functional. Furthermore, it provides the basis for the development of a process for NeuNAc production as well as a general prospective design for production processes that use saprophytes as whole-cell catalysts.

Stable coexpression of two human sialylation enzymes in plant suspension-cultured tobacco cells

Human CMP-N-acetylneuraminic acid (NeuAc) synthase (hCSS) and α2,6-sialyltransferase (hST) participate in the sialylation of N-linked glycans in mammalian cells. hCSS synthesizes CMP-NeuAc, which hST uses as a donor substrate to transfer NeuAc to the terminal position of N-linked glycans. In plant cells, the presence of NeuAc has not yet been substantiated and the identification of the genes involved in the sialylation of N-glycan has not been carried out. In this study, we introduced hCSS and hST genes into suspension-cultured tobacco BY2 cells to provide the machinery for the sialylation pathway in plants. hCSS and hST stably expressed in the plant cells showed activity. In addition, CMP-NeuAc produced by hCSS in the transformed plant cells functioned as a donor substrate to hST. An in vitro coupled hCSS and hST reaction resulted in the production of mammalian-type sialoglycoproteins bearing terminal NeuAc residues. Furthermore, the results of the purification of the coupled-reaction products by Sambucus sieboldian lectin column chromatography and digestion with linkage-specific neuraminidase revealed that the modified terminal residue was α2,6-linked NeuAc. Here, we demonstrate that the in vitro sialylation of N-linked glycans on mammalian proteins can be achieved using plant cell extracts stably expressing hCSS and hST, providing proof-of-principle that a sialylated human therapeutic protein can be produced in plants.

Plant-derived vaccines and other therapeutics produced in contained systems

The use of contained plant systems for the production of biopharmaceuticals represents a powerful alternative to current methods, combining the benefits of whole-plant systems and cell cultures. In vitro contained production systems include plant cell suspensions, hairy root cultures, novel plants grown in contained conditions and microalgae. These systems show intrinsic advantages, such as control over growth conditions, production in compliance with good manufacturing practice and avoidance of political resistance to the release of genetically modified field crops. At present, one of the two plant-produced vaccine-related products that have gone all the way through production and regulatory hurdles derives from tobacco cell suspensions, and the second is a human therapeutic enzyme, which is expected to reach commercial development soon and derives from carrot suspension cells. In the future, several other products from contained systems are expected to reach the clinical trial stage.

Manufacturing antibodies in the plant cell

Plants have long been considered advantageous platforms for large-scale production of antibodies due to their low cost, scalability, and the low chances of pathogen contamination. Much effort has therefore been devoted to efficiently producing mAbs (from nanobodies to secretory antibodies) in plant cells. Several technical difficulties have been encountered and are being overcome. Improvements in production levels have been achieved by manipulation of gene expression and, more efficiently, of cell targeting and protein folding and assembly. Differences in mAb glycosylation patterns between animal and plant cells are being successfully addressed by the elimination and introduction of the appropriate enzyme activities in plant cells. Another relevant battlefield is the dichotomy between production capacity and speed. Classically, stably transformed plant lines have been proposed for large scale mAb production, whereas the use of transient expression systems has always provided production speed at the cost of scalability. However, recent advances in transient expression techniques have brought impressive yield improvements, turning speed and scalability into highly compatible assets. In the era of personalized medicines, the combination of yield and speed, and the advances in glyco-engineering have made the plant cell a serious contender in the field of recombinant antibody production.

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.

From Neanderthal to nanobiotech: from plant potions to pharming with plant factories

Plants were the main source for human drugs until the beginning of the nineteenth century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. During the last decades of the twentieth century, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. After a temporary decrease in interest, plants are rapidly moving back into human pharmacopoeia, with the recent development of plant-based recombinant protein production systems offering a safe and extremely cost-effective alternative to microbial and mammalian cell cultures. In this short review, we will illustrate that current improvements in plant expression systems are making them suitable as alternative factories for the production of either simple or highly complex therapeutic proteins.

Down-regulated expression of plant-specific glycoepitopes in alfalfa

SUMMARY

Compared with other plant expression systems used for pharmaceutical protein production, alfalfa offers the advantage of very homogeneous N-glycosylation. Therefore, this plant was selected for further attempts at glycoengineering. Two main approaches were developed in order to humanize N-glycosylation in alfalfa. The first was a knock-down of two plant-specific N-glycan maturation enzymes, beta1,2-xylosyltransferase and alpha1,3-fucosyltransferases, using sense, antisense and RNA interference strategies. In a second approach, with the ultimate goal of rebuilding the whole human sialylation pathway, human beta1,4-galactosyltransferase was expressed in alfalfa in a native form or in fusion with a targeting domain from N-acetylglucosaminyltransferase I, a glycosyltransferase located in the early Golgi apparatus in Nicotiana tabacum. Both knock-down and knock-in strategies strongly, but not completely, inhibited the biosynthesis of alpha1,3-fucose- and beta1,2-xylose-containing glycoepitopes in transgenic alfalfa. However, recombinant human beta1,4-galactosyltransferase activity in transgenic alfalfa completely prevented the accumulation of the Lewis a glycoepitope on complex N-glycans.

Plant N-glycan profiling of minute amounts of material

Development of convenient strategies for identification of plant N-glycan profiles has been driven by the emergence of plants as an expression system for therapeutic proteins. In this article, we reinvestigated qualitative and quantitative aspects of plant N-glycan profiling. The extraction of plant proteins through a phenol/ammonium acetate procedure followed by deglycosylation with peptide N-glycosidase A (PNGase A) and coupling to 2-aminobenzamide provides an oligosaccharide preparation containing reduced amounts of contaminants from plant cell wall polysaccharides. Such a preparation was also suitable for accurate qualitative and quantitative evaluation of the N-glycan content by mass spectrometry. Combining these approaches allows the profiling to be carried out from as low as 500 mg of fresh leaf material. We also demonstrated that collision-induced dissociation (CID) mass spectrometry in negative mode of N-glycans harboring alpha(1,3)- or alpha(1,6)-fucose residue on the proximal GlcNAc leads to specific fragmentation patterns, thereby allowing the discrimination of plant N-glycans from those arising from mammalian contamination.

Moss bioreactors producing improved biopharmaceuticals

Plants may serve as superior production systems for complex recombinant pharmaceuticals. Current strategies for improving plant-based systems include the development of large-scale production facilities as well as the optimisation of protein modifications. While post-translational modifications of plant proteins generally resemble those of mammalian proteins, certain plant-specific protein-linked sugars are immunogenic in humans, a fact that restricts the use of plants in biopharmaceutical production so far. The moss Physcomitrella patens was developed as a contained tissue culture system for recombinant protein production in photo-bioreactors. By targeted gene replacements, moss strains were created with non-immunogenic humanised glycan patterns. These were proven to be superior to currently used mammalian cell lines in producing antibodies with enhanced effectiveness.

A human anti-Pseudomonas aeruginosa serotype O6ad immunoglobulin G1 expressed in transgenic tobacco is capable of recruiting immune system effector function in vitro

The production of a recombinant human IgG1 in transgenic tobacco was examined to determine whether a plant-derived antibody could recruit immune system effector function against a bacterial pathogen. A plant transformation vector was engineered to contain genes for a human kappa light chain and a human gamma-1 heavy chain with V(H) and V(L) sequences from a previously identified human IgG2 monoclonal antibody (MAb) that specifically binds to and opsonizes Pseudomonas aeruginosa serotype O6ad. Unique NcoI and NotI restriction sites were incorporated to flank these variable sequences, resulting in a plant transformation vector that could be engineered for expression of any other human IgG1 antibody, requiring only the substitution of other V(H) and V(L) antigen-binding coding sequences. The plant-produced IgG1 was determined to have high-mannose glycan content and to be capable of mediating opsonophagocytosis of P. aeruginosa serotype O6ad in vitro using human complement and human polymorphonuclear leukocytes. Thus, MAbs produced in plants from this vector could provide human IgG1 MAbs for targeting other pathogens that require the recruitment of immune system effector functions.

From planta to pharma with glycosylation in the toolbox

Plant-specific glycosylation has long been a major limitation to the extensive use of plant-made pharmaceuticals in human therapy. Our goal here is to highlight the progress recently made towards humanization of N-glycosylation in plants and to illustrate that plant-typical N- and O-glycosylation progressively emerge as additional advantages for using this promising expression system.

Pharming and transgenic plants

Plant represented the essence of pharmacopoeia until the beginning of the 19th century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. In the last decades, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. More recently, molecular farming has rapidly pushed towards plants among the major players in recombinant protein production systems. Indeed, therapeutic protein production is safe and extremely cost-effective in plants. Unlike microbial fermentation, plants are capable of carrying out post-translational modifications and, unlike production systems based on mammalian cell cultures, plants are devoid of human infective viruses and prions. Furthermore, a large panel of strategies and new plant expression systems are currently developed to improve the plant-made pharmaceutical's yields and quality. Recent advances in the control of post-translational maturations in transgenic plants will allow them, in the near future, to perform human-like maturations on recombinant proteins and, hence, make plant expression systems suitable alternatives to animal cell factories.