Engineering mammalian mucin-type O-glycosylation in plants

Implications of O-glycan modifications in the hinge region of a plant-produced SARS-CoV-2-IgA antibody on functionality

Introduction: Prolyl-4-hydroxylases (P4H) catalyse the irreversible conversion of proline to hydroxyproline, constituting a common posttranslational modification of proteins found in humans, plants, and microbes. Hydroxyproline residues can be further modified in plants to yield glycoproteins containing characteristic O-glycans. It is currently unknown how these plant endogenous modifications impact protein functionality and they cause considerable concerns for the recombinant production of therapeutic proteins in plants. In this study, we carried out host engineering to generate a therapeutic glycoprotein largely devoid of plant-endogenous O-glycans for functional characterization. Methods: Genome editing was used to inactivate two genes coding for enzymes of the P4H10 subfamily in the widely used expression host Nicotiana benthamiana. Using glycoengineering in plants and expression in human HEK293 cells we generated four variants of a potent, SARS-CoV-2 neutralizing antibody, COVA2-15 IgA1. The variants that differed in the number of modified proline residues and O-glycan compositions of their hinge region were assessed regarding their physicochemical properties and functionality. Results: We found that plant endogenous O-glycan formation was strongly reduced on IgA1 when transiently expressed in the P4H10 double mutant N. benthamiana plant line. The IgA1 glycoforms displayed differences in proteolytic stability and minor differences in receptor binding thus highlighting the importance of O-glycosylation in the hinge region of human IgA1. Discussion: This work reports the successful protein O-glycan engineering of an important plant host for recombinant protein expression. While the complete removal of endogenous hydroxyproline residues from the hinge region of plant-produced IgA1 is yet to be achieved, our engineered line is suitable for structure-function studies of O-glycosylated recombinant glycoproteins produced in plants.

Plant glycoengineering for designing next-generation vaccines and therapeutic proteins

Protein glycosylation has a huge impact on biological processes in all domains of life. The type of glycan present on a recombinant glycoprotein depends on protein intrinsic features and the glycosylation repertoire of the cell type used for expression. Glycoengineering approaches are used to eliminate unwanted glycan modifications and to facilitate the coordinated expression of glycosylation enzymes or whole metabolic pathways to furnish glycans with distinct modifications. The formation of tailored glycans enables structure-function studies and optimization of therapeutic proteins used in different applications. While recombinant proteins or proteins from natural sources can be in vitro glycoengineered using glycosyltransferases or chemoenzymatic synthesis, many approaches use genetic engineering involving the elimination of endogenous genes and introduction of heterologous genes to cell-based production systems. Plant glycoengineering enables the in planta production of recombinant glycoproteins with human or animal-type glycans that resemble natural glycosylation or contain novel glycan structures. This review summarizes key achievements in glycoengineering of plants and highlights current developments aiming to make plants more suitable for the production of a diverse range of recombinant glycoproteins for innovative therapies.

Recombinant mucin biotechnology and engineering

Mucins represent a largely untapped class of polymeric building block for biomaterials, therapeutics, and other biotechnology. Because the mucin polymer backbone is genetically encoded, sequence-specific mucins with defined physical and biochemical properties can be fabricated using recombinant technologies. The pendent O-glycans of mucins are increasingly implicated in immunomodulation, suppression of pathogen virulence, and other biochemical activities. Recent advances in engineered cell production systems are enabling the scalable synthesis of recombinant mucins with precisely tuned glycan side chains, offering exciting possibilities to tune the biological functionality of mucin-based products. New metabolic and chemoenzymatic strategies enable further tuning and functionalization of mucin O-glycans, opening new possibilities to expand the chemical diversity and functionality of mucin building blocks. In this review, we discuss these advances, and the opportunities for engineered mucins in biomedical applications ranging from in vitro models to therapeutics.

Glyco-Engineering Plants to Produce Helminth Glycoproteins as Prospective Biopharmaceuticals: Recent Advances, Challenges and Future Prospects

Glycoproteins are the dominant category among approved biopharmaceuticals, indicating their importance as therapeutic proteins. Glycoproteins are decorated with carbohydrate structures (or glycans) in a process called glycosylation. Glycosylation is a post-translational modification that is present in all kingdoms of life, albeit with differences in core modifications, terminal glycan structures, and incorporation of different sugar residues. Glycans play pivotal roles in many biological processes and can impact the efficacy of therapeutic glycoproteins. The majority of biopharmaceuticals are based on human glycoproteins, but non-human glycoproteins, originating from for instance parasitic worms (helminths), form an untapped pool of potential therapeutics for immune-related diseases and vaccine candidates. The production of sufficient quantities of correctly glycosylated putative therapeutic helminth proteins is often challenging and requires extensive engineering of the glycosylation pathway. Therefore, a flexible glycoprotein production system is required that allows straightforward introduction of heterologous glycosylation machinery composed of glycosyltransferases and glycosidases to obtain desired glycan structures. The glycome of plants creates an ideal starting point for N- and O-glyco-engineering of helminth glycans. Plants are also tolerant toward the introduction of heterologous glycosylation enzymes as well as the obtained glycans. Thus, a potent production platform emerges that enables the production of recombinant helminth proteins with unusual glycans. In this review, we discuss recent advances in plant glyco-engineering of potentially therapeutic helminth glycoproteins, challenges and their future prospects.

Transient Expression of Glycosylated SARS-CoV-2 Antigens in Nicotiana benthamiana

The current COVID-19 pandemic very dramatically shows that the world lacks preparedness for novel viral diseases. In addition to newly emerging viruses, many known pathogenic viruses such as influenza are constantly evolving, leading to frequent outbreaks with severe diseases and deaths. Hence, infectious viruses are a recurrent burden to our daily life, and powerful strategies to stop the spread of human pathogens and disease progression are of utmost importance. Transient plant-based protein expression is a technology that allows fast and highly flexible manufacturing of recombinant viral proteins and, thus, can contribute to infectious disease detection and prevention. This review highlights recent progress in the transient production of viral glycoproteins in N. benthamiana with a focus on SARS-CoV-2-derived viral antigens.

Prolyl Hydroxylase Paralogs in Nicotiana benthamiana Show High Similarity With Regard to Substrate Specificity

Plant glycoproteins display a characteristic type of O-glycosylation where short arabinans or larger arabinogalactans are linked to hydroxyproline. The conversion of proline to 4-hydroxyproline is accomplished by prolyl-hydroxylases (P4Hs). Eleven putative Nicotiana benthamiana P4Hs, which fall in four homology groups, have been identified by homology searches using known Arabidopsis thaliana P4H sequences. One member of each of these groups has been expressed in insect cells using the baculovirus expression system and applied to synthetic peptides representing the O-glycosylated region of erythropoietin (EPO), IgA1, Art v 1 and the Arabidopsis thaliana glycoprotein STRUBBELIG. Unlike the situation in the moss Physcomitrella patens, where one particular P4H was mainly responsible for the oxidation of erythropoietin, the tobacco P4Hs exhibited rather similar activities, albeit with biased substrate preferences and preferred sites of oxidation. From a biotechnological viewpoint, this result means that silencing/knockout of a single P4H in N. benthamiana cannot be expected to result in the abolishment of the plant-specific oxidation of prolyl residues in a recombinant protein.

β-Hexosaminidases Along the Secretory Pathway of Nicotiana benthamiana Have Distinct Specificities Toward Engineered Helminth N-Glycans on Recombinant Glycoproteins

Secretions of parasitic worms (helminths) contain a wide collection of immunomodulatory glycoproteins with the potential to treat inflammatory disorders, like autoimmune diseases. Yet, the identification of single molecules that can be developed into novel biopharmaceuticals is hampered by the limited availability of native parasite-derived proteins. Recently, pioneering work has shown that helminth glycoproteins can be produced transiently in Nicotiana benthamiana plants while simultaneously mimicking their native helminth N-glycan composition by co-expression of desired glycosyltransferases. However, efficient "helminthization" of N-glycans in plants by glyco-engineering seems to be hampered by the undesired truncation of complex N-glycans by β-N-acetyl-hexosaminidases, in particular when aiming for the synthesis of N-glycans with antennary GalNAcβ1-4GlcNAc (LacdiNAc or LDN). In this study, we cloned novel β-hexosaminidase open reading frames from N. benthamiana and characterized the biochemical activity of these enzymes. We identified HEXO2 and HEXO3 as enzymes responsible for the cleavage of antennary GalNAc residues of N-glycans on the model helminth glycoprotein kappa-5. Furthermore, we reveal that each member of the HEXO family has a distinct specificity for N-glycan substrates, where HEXO2 has strict β-galactosaminidase activity, whereas HEXO3 cleaves both GlcNAc and GalNAc. The identification of HEXO2 and HEXO3 as major targets for LDN cleavage will enable a targeted genome editing approach to reduce undesired processing of these N-glycans. Effective knockout of these enzymes could allow the production of therapeutically relevant glycoproteins with tailor-made helminth N-glycans in plants.

Glycosylation of Plant-Produced Immunoglobulins

Many economically important protein-based therapeutics like monoclonal antibodies are glycosylated. Due to the recognized importance of this type of posttranslational modification, glycoengineering of expression systems to obtain highly active and homogenous therapeutics is an emerging field. Although most of the monoclonal antibodies on the market are still produced in mammalian expression platforms, plants are emerging as an alternative cost-effective and scalable production platform that allows precise engineering of glycosylation to produce targeted human glycoforms at large homogeneity. Apart from producing more effective antibodies, pure glycoforms are required in efforts to link biological functions to specific glycan structures. Much is already known about the role of IgG1 glycosylation and this antibody class is the dominant recombinant format that has been expressed in plants. By contrast, little attention has been paid to the glycoengineering of recombinant IgG subtypes and the other four classes of human immunoglobulins (IgA, IgD, IgE, and IgM). Except for IgD, all these antibody classes have been expressed in plants and the glycosylation has been analyzed in a site-specific manner. Here, we summarize the current data on glycosylation of plant-produced monoclonal antibodies and discuss the findings in the light of known functions for these glycans.

A Roadmap for the Molecular Farming of Viral Glycoprotein Vaccines: Engineering Glycosylation and Glycosylation-Directed Folding

Immunization with recombinant glycoprotein-based vaccines is a promising approach to induce protective immunity against viruses. However, the complex biosynthetic maturation requirements of these glycoproteins typically necessitate their production in mammalian cells to support their folding and post-translational modification. Despite these clear advantages, the incumbent costs and infrastructure requirements with this approach can be prohibitive in developing countries, and the production scales and timelines may prove limiting when applying these production systems to the control of pandemic viral outbreaks. Plant molecular farming of viral glycoproteins has been suggested as a cheap and rapidly scalable alternative production system, with the potential to perform post-translational modifications that are comparable to mammalian cells. Consequently, plant-produced glycoprotein vaccines for seasonal and pandemic influenza have shown promise in clinical trials, and vaccine candidates against the newly emergent severe acute respiratory syndrome coronavirus-2 have entered into late stage preclinical and clinical testing. However, many other viral glycoproteins accumulate poorly in plants, and are not appropriately processed along the secretory pathway due to differences in the host cellular machinery. Furthermore, plant-derived glycoproteins often contain glycoforms that are antigenically distinct from those present on the native virus, and may also be under-glycosylated in some instances. Recent advances in the field have increased the complexity and yields of biologics that can be produced in plants, and have now enabled the expression of many viral glycoproteins which could not previously be produced in plant systems. In contrast to the empirical optimization that predominated during the early years of molecular farming, the next generation of plant-made products are being produced by developing rational, tailor-made approaches to support their production. This has involved the elimination of plant-specific glycoforms and the introduction into plants of elements of the biosynthetic machinery from different expression hosts. These approaches have resulted in the production of mammalian N-linked glycans and the formation of O-glycan moieties in planta. More recently, plant molecular engineering approaches have also been applied to improve the glycan occupancy of proteins which are not appropriately glycosylated, and to support the folding and processing of viral glycoproteins where the cellular machinery differs from the usual expression host of the protein. Here we highlight recent achievements and remaining challenges in glycoengineering and the engineering of glycosylation-directed folding pathways in plants, and discuss how these can be applied to produce recombinant viral glycoproteins vaccines.

Partial purification and immunodetection of cell surface glycoproteins from plants

Cell surface glycoproteins in plants were first described more than 50 years ago, and yet, the precise mechanisms by which they operate remain elusive to this day. Studying glycoproteins is often challenging due to their subcellular localization (many secreted or membrane associated) and the extent of glycosylation present on the protein backbone, which can have profound effects on protein structure and behavior. In plants, additional layers of complexity exist as cell surface glycoproteins are in close contact, and in some cases, establish direct linkages with the polysaccharide networks present in the cell wall. In this chapter, we guide the reader through a protocol aimed to address the glycosylation status of a presumed cell surface glycoprotein. First, we discuss the advantages and disadvantages of using plants as homologous expression systems for recombinant glycoprotein production. Next, we describe a protocol for microsomal enrichment, followed by partial purification by affinity chromatography and finally glycodetection by immunoblotting using monoclonal antibodies targeting cell wall glycans. We particularly focus on the hydroxyproline-rich glycoprotein (HRGP) family, the most abundant family of glycoproteins in the plant cell wall. We provide examples of two putative HRGP chimeric proteins, one akin to extensins and the second an arabinogalactan protein (AGP)-like protein. For the latter, we provide an AGP-specific protocol to ensure enrichment of members of this group, which can be used independently or in conjunction with the described protocol. Throughout the chapter, we provide recommendations for the handling of plant glycoproteins and highlight special considerations for experimental design, along with troubleshooting suggestions.

Engineering the Plant Secretory Pathway for the Production of Next-Generation Pharmaceuticals

Production of biologics in plants, or plant molecular pharming, is a promising protein expression technology that is receiving increasing attention from the pharmaceutical industry. Previously, low expression yields of recombinant proteins and the realization that certain post-translational modifications (PTMs) may not occur optimally limited the widespread acceptance of the technology. However, molecular engineering of the plant secretory pathway is now enabling the production of increasingly complex biomolecules using tailored protein-specific approaches to ensure their maturation. These involve the elimination of undesired processing events, and the introduction of heterologous biosynthetic machinery to support the production of specific target proteins. Here, we discuss recent advances in the production of pharmaceutical proteins in plants, which leverage the unique advantages of the technology.

Engineering orthogonal human O-linked glycoprotein biosynthesis in bacteria

A major objective of synthetic glycobiology is to re-engineer existing cellular glycosylation pathways from the top down or construct non-natural ones from the bottom up for new and useful purposes. Here, we have developed a set of orthogonal pathways for eukaryotic O-linked protein glycosylation in Escherichia coli that installed the cancer-associated mucin-type glycans Tn, T, sialyl-Tn and sialyl-T onto serine residues in acceptor motifs derived from different human O-glycoproteins. These same glycoengineered bacteria were used to supply crude cell extracts enriched with glycosylation machinery that permitted cell-free construction of O-glycoproteins in a one-pot reaction. In addition, O-glycosylation-competent bacteria were able to generate an antigenically authentic Tn-MUC1 glycoform that exhibited reactivity with antibody 5E5, which specifically recognizes cancer-associated glycoforms of MUC1. We anticipate that the orthogonal glycoprotein biosynthesis pathways developed here will provide facile access to structurally diverse O-glycoforms for a range of important scientific and therapeutic applications.

Synthetic Glycobiology: Parts, Systems, and Applications

Protein glycosylation, the attachment of sugars to amino acid side chains, can endow proteins with a wide variety of properties of great interest to the engineering biology community. However, natural glycosylation systems are limited in the diversity of glycoproteins they can synthesize, the scale at which they can be harnessed for biotechnology, and the homogeneity of glycoprotein structures they can produce. Here we provide an overview of the emerging field of synthetic glycobiology, the application of synthetic biology tools and design principles to better understand and engineer glycosylation. Specifically, we focus on how the biosynthetic and analytical tools of synthetic biology have been used to redesign glycosylation systems to obtain defined glycosylation structures on proteins for diverse applications in medicine, materials, and diagnostics. We review the key biological parts available to synthetic biologists interested in engineering glycoproteins to solve compelling problems in glycoscience, describe recent efforts to construct synthetic glycoprotein synthesis systems, and outline exemplary applications as well as new opportunities in this emerging space.

Engineering mammalian cells to produce plant-specific N-glycosylation on proteins

Protein N-glycosylation is an essential and highly conserved posttranslational modification found in all eukaryotic cells. Yeast, plants and mammalian cells, however, produce N-glycans with distinct structural features. These species-specific features not only pose challenges in selecting host cells for production of recombinant therapeutics for human medical use but also provide opportunities to explore and utilize species-specific glycosylation in design of vaccines. Here, we used reverse cross-species engineering to stably introduce plant core α3fucose (α3Fuc) and β2xylose (β2Xyl) N-glycosylation epitopes in the mammalian Chinese hamster ovary (CHO) cell line. We used directed knockin of plant core fucosylation and xylosylation genes (AtFucTA/AtFucTB and AtXylT) and targeted knockout of endogenous genes for core fucosylation (fut8) and elongation (B4galt1), for establishing CHO cells with plant N-glycosylation capacities. The engineering was evaluated through coexpression of two human therapeutic N-glycoproteins, erythropoietin (EPO) and an immunoglobulin G (IgG) antibody. Full conversion to the plant-type α3Fuc/β2Xyl bi-antennary agalactosylated N-glycosylation (G0FX) was demonstrated for the IgG1 produced in CHO cells. These results demonstrate that N-glycosylation in mammalian cells is amenable for extensive cross-kingdom engineering and that engineered CHO cells may be used to produce glycoproteins with plant glycosylation.

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.

A Bacterial Expression Platform for Production of Therapeutic Proteins Containing Human-like O-Linked Glycans

We have developed an Escherichia coli strain for the in vivo production of O-glycosylated proteins. This was achieved using a dual plasmid approach: one encoding a therapeutic protein target, and a second encoding the enzymatic machinery required for O-glycosylation. The latter plasmid encodes human polypeptide N-acetylgalactosaminyl transferase as well as a β1,3-galactosyl transferase and UDP-Glc(NAc)-4-epimerase, both from Campylobacter jejuni, and a disulfide bond isomerase of bacterial or human origin. The effectiveness of this two-plasmid synthetic operon system has been tested on three proteins with therapeutic potential: the native and an engineered version of the naturally O-glycosylated human interferon α-2b, as well as human growth hormone with one engineered site of glycosylation. Having established proof of principle for the addition of the core-1 glycan onto proteins, we are now developing this system as a platform for producing and modifying human protein therapeutics with more complex O-glycan structures in E. coli.

Advanced Plant-Based Glycan Engineering

With respect to biomanufacturing, glycosylation is one of the most addressed post-translational modifications, since it is well-known that the attachment of sugar residues efficiently affects protein homogeneity and functionality. Much effort has been taken into engineering various expression systems to control glycosylation and to generate molecules with targeted sugar profiles. Nevertheless, engineering of N- and O-linked glycans on well-established expression systems remains challenging. On the one side the glycosylation machinery in mammalian cells is hard to control due to its complexity. Most bacteria, on the other side, completely lack such glycan formations, and in general exhibit fundamental differences in their glycosylation abilities. Beyond that, plants generate complex N-glycans typical of higher eukaryotes, but simpler than those produced by mammals. Paradoxically, it seems that the limited glycosylation capacity of plant cells is an advantage for specific glycan manipulations. This review focuses on recent achievements in plant glycan engineering and provides a short outlook on how new developments (in synthetic biology) might have a positive impact.

Main Strategies of Plant Expression System Glycoengineering for Producing Humanized Recombinant Pharmaceutical Proteins

Most the pharmaceutical proteins are derived not from their natural sources, rather their recombinant analogs are synthesized in various expression systems. Plant expression systems, unlike mammalian cell cultures, combine simplicity and low cost of procaryotic systems and the ability for posttranslational modifications inherent in eucaryotes. More than 50% of all human proteins and more than 40% of the currently used pharmaceutical proteins are glycosylated, that is, they are glycoproteins, and their biological activity, pharmacodynamics, and immunogenicity depend on the correct glycosylation pattern. This review examines in detail the similarities and differences between N- and O-glycosylation in plant and mammalian cells, as well as the effect of plant glycans on the activity, pharmacokinetics, immunity, and intensity of biosynthesis of pharmaceutical proteins. The main current strategies of glycoengineering of plant expression systems aimed at obtaining fully humanized proteins for pharmaceutical application are summarized.

Transient co-expression with three O-glycosylation enzymes allows production of GalNAc-O-glycosylated Granulocyte-Colony Stimulating Factor in N. benthamiana

BACKGROUND

Expression of economically relevant proteins in alternative expression platforms, especially plant expression platforms, has gained significant interest in recent years. A special interest in working with plants as bioreactors for the production of pharmaceutical proteins is related to low production costs, product safety and quality. Among the different properties that plants can also offer for the production of recombinant proteins, protein glycosylation is crucial since it may have an impact on pharmaceutical functionality and/or stability.

RESULTS

The pharmaceutical glycoprotein human Granulocyte-Colony Stimulating Factor was transiently expressed in Nicotiana benthamiana plants and subjected to mammalian-specific mucin-type O-glycosylation by co-expressing the pharmaceutical protein together with the glycosylation machinery responsible for such post-translational modification.

CONCLUSIONS

The pharmaceutical glycoprotein human Granulocyte-Colony Stimulating Factor can be expressed in N. benthamiana plants via agroinfiltration with its native mammalian-specific mucin-type O-glycosylation.

Exploring Site-Specific N-Glycosylation of HEK293 and Plant-Produced Human IgA Isotypes

The full potential of recombinant Immunoglobulin A as therapeutic antibody is not fully explored, owing to the fact that structure-function relationships of these extensively glycosylated proteins are not well understood. Here monomeric IgA1, IgA2m(1), and IgA2m(2) variants of the anti-HER2 antibody (IgG1) trastuzumab were expressed in glyco-engineered Nicotiana benthamiana plants and in human HEK293-6E cells. All three IgA isotypes were purified and subjected to biophysical and biochemical characterization. While no differences in assembly, antigen binding, and glycosylation occupancy were observed, both systems vary tremendously in terms of glycan structures and heterogeneity of glycosylation. Mass-spectrometric analysis of site-specific glycosylation revealed that plant-produced IgAs carry mainly complex-type biantennary N-glycans. HEK293-6E-produced IgAs, on the contrary, showed very heterogeneous N-glycans with high levels of sialylation, core-fucose, and the presence of branched structures. The site-specific analysis revealed major differences between the individual N-glycosylation sites of each IgA subtype. Moreover, the proline-rich hinge region from HEK293-6E cell-derived IgA1 was occupied with mucin-type O-glycans, whereas IgA1 from N. benthamiana displayed numerous plant-specific modifications. Interestingly, a shift in unfolding of the CH2 domain of plant-produced IgA toward lower temperatures can be observed with differential scanning calorimetry, suggesting that distinct glycoforms affect the thermal stability of IgAs.

Plant glyco-biotechnology

Glycosylation is an important protein modification in all eukaryotes. Whereas the early asparagine-linked glycosylation (N-glycosylation) and N-glycan processing steps in the endoplasmic reticulum are conserved between mammals and plants, the maturation of complex N-glycans in the Golgi apparatus differs considerably. Due to a restricted number of Golgi-resident N-glycan processing enzymes and the absence of nucleotide sugars such as CMP-N-acetylneuraminic acid, plants produce only a limited repertoire of different N-glycan structures. Moreover, mammalian mucin-type O-glycosylation of serine or threonine residues has not been described in plants and the required machinery is not encoded in their genome which enables de novo build-up of the pathway. As a consequence, plants are very well-suited for the production of homogenous N- and O-glycans and are increasingly used for the production of recombinant glycoproteins with custom-made glycans that may result in the generation of biopharmaceuticals with improved therapeutic potential.

Production and glyco-engineering of immunomodulatory helminth glycoproteins in plants

Helminth parasites control host-immune responses by secreting immunomodulatory glycoproteins. Clinical trials and mouse model studies have demonstrated the potential of helminth-derived glycoproteins for the treatment of immune-related diseases, like allergies and autoimmune diseases. Studies are however hampered by the limited availability of native parasite-derived proteins. Moreover, recombinant protein production systems have thus far been unable to reconstitute helminth-like glycosylation essential for the functionality of some helminth glycoproteins. Here we exploited the flexibility of the N-glycosylation machinery of plants to reconstruct the helminth glycoproteins omega-1 and kappa-5, two major constituents of immunomodulatory Schistosoma mansoni soluble egg antigens. Fine-tuning transient co-expression of specific glycosyltransferases in Nicotiana benthamiana enabled the synthesis of Lewis X (LeX) and LDN/LDN-F glycan motifs as found on natural omega-1 and kappa-5, respectively. In vitro and in vivo evaluation of the introduction of native LeX motifs on plant-produced omega-1 confirmed that LeX on omega-1 contributes to the glycoprotein's Th2-inducing properties. These data indicate that mimicking the complex carbohydrate structures of helminths in plants is a promising strategy to allow targeted evaluation of therapeutic glycoproteins for the treatment of inflammatory disorders. In addition, our results offer perspectives for the development of effective anti-helminthic vaccines by reconstructing native parasite glycoprotein antigens.

Antibody Production in Plants and Green Algae

Monoclonal antibodies (mAbs) have a wide range of modern applications, including research, diagnostic, therapeutic, and industrial uses. Market demand for mAbs is high and continues to grow. Although mammalian systems, which currently dominate the biomanufacturing industry, produce effective and safe recombinant mAbs, they have a limited manufacturing capacity and high costs. Bacteria, yeast, and insect cell systems are highly scalable and cost effective but vary in their ability to produce appropriate posttranslationally modified mAbs. Plants and green algae are emerging as promising production platforms because of their time and cost efficiencies, scalability, lack of mammalian pathogens, and eukaryotic posttranslational protein modification machinery. So far, plant- and algae-derived mAbs have been produced predominantly as candidate therapeutics for infectious diseases and cancer. These candidates have been extensively evaluated in animal models, and some have shown efficacy in clinical trials. Here, we review ongoing efforts to advance the production of mAbs in plants and algae.

The case for plant-made veterinary immunotherapeutics

The excessive use of antibiotics in food animal production has contributed to resistance in pathogenic bacteria, thereby triggering regulations and consumer demands to limit their use. Alternatives for disease control are therefore required that are cost-effective and compatible with intensive production. While vaccines are widely used and effective, they are available against a minority of animal diseases, and development of novel vaccines and other immunotherapeutics is therefore needed. Production of such proteins recombinantly in plants can provide products that are effective and safe, can be orally administered with minimal processing, and are easily scalable with a relatively low capital investment. The present report thus advocates the use of plants for producing vaccines and antibodies to protect farm animals from diseases that have thus far been managed with antibiotics; and highlights recent advances in product efficacy, competitiveness, and regulatory approval.

Two key polymorphisms in a newly discovered allele of the Vitis vinifera TPS24 gene are responsible for the production of the rotundone precursor α-guaiene

Rotundone was initially identified as a grape-derived compound responsible for the peppery aroma of Shiraz wine varieties. It has subsequently been found in black and white pepper and several other spices. Because of its potent aroma, the molecular basis for rotundone formation is of particular relevance to grape and wine scientists and industry. We have identified and functionally characterized in planta a sesquiterpene synthase, VvGuaS, from developing grape berries, and have demonstrated that it produces the precursor of rotundone, α-guaiene, as its main product. The VvGuaS enzyme is a novel allele of the sesquiterpene synthase gene, VvTPS24, which has previously been reported to encode VvPNSeInt, an enzyme that produces a variety of selinene-type sesquiterpenes. This newly discovered VvTPS24 allele encodes an enzyme 99.5% identical to VvPNSeInt, with the differences comprising just 6 out of the 561 amino acid residues. Molecular modelling of the enzymes revealed that two of these residues, T414 and V530, are located in the active site of VvGuaS within 4 Å of the binding-site of the substrate, farnesyl pyrophosphate. Mutation of these two residues of VvGuaS into the corresponding polymorphisms in VvPNSeInt results in a complete functional conversion of one enzyme into the other, while mutation of each residue individually produces an intermediate change in the product profile. We have therefore demonstrated that VvGuaS, an enzyme responsible for production of the rotundone precursor, α-guaiene, is encoded by a novel allele of the previously characterized grapevine gene VvTPS24 and that two specific polymorphisms are responsible for functional differences between VvTPS24 alleles.

Transient Glyco-Engineering to Produce Recombinant IgA1 with Defined N- and O-Glycans in Plants

The production of therapeutic antibodies to combat pathogens and treat diseases, such as cancer is of great interest for the biotechnology industry. The recent development of plant-based expression systems has demonstrated that plants are well-suited for the production of recombinant monoclonal antibodies with defined glycosylation. Compared to immunoglobulin G (IgG), less effort has been undertaken to express immunoglobulin A (IgA), which is the most prevalent antibody class at mucosal sites and a promising candidate for novel recombinant biopharmaceuticals with enhanced anti-tumor activity. Here, we transiently expressed recombinant human IgA1 against the VP8* rotavirus antigen in glyco-engineered ΔXT/FT Nicotiana benthamiana plants. Mass spectrometric analysis of IgA1 glycopeptides revealed the presence of complex biantennary N-glycans with terminal N-acetylglucosamine present on the N-glycosylation site of the CH2 domain in the IgA1 alpha chain. Analysis of the peptide carrying nine potential O-glycosylation sites in the IgA1 alpha chain hinge region showed the presence of plant-specific modifications including hydroxyproline formation and the attachment of pentoses. By co-expression of enzymes required for initiation and elongation of human O-glycosylation it was possible to generate disialylated mucin-type core 1 O-glycans on plant-produced IgA1. Our data demonstrate that ΔXT/FT N. benthamiana plants can be engineered toward the production of recombinant IgA1 with defined human-type N- and O-linked glycans.

Using glyco-engineering to produce therapeutic proteins

INTRODUCTION

Glycans are increasingly important in the development of new biopharmaceuticals with optimized efficacy, half-life, and antigenicity. Current expression platforms for recombinant glycoprotein therapeutics typically do not produce homogeneous glycans and frequently display non-human glycans which may cause unwanted side effects. To circumvent these issues, glyco-engineering has been applied to different expression systems including mammalian cells, insect cells, yeast, and plants.

AREAS COVERED

This review summarizes recent developments in glyco-engineering focusing mainly on in vivo expression systems for recombinant proteins. The highlighted strategies aim at producing glycoproteins with homogeneous N- and O-linked glycans of defined composition.

EXPERT OPINION

Glyco-engineering of expression platforms is increasingly recognized as an important strategy to improve biopharmaceuticals. A better understanding and control of the factors leading to glycan heterogeneity will allow simplified production of recombinant glycoprotein therapeutics with less variation in terms of glycosylation. Further technological advances will have a major impact on manufacturing processes and may provide a completely new class of glycoprotein therapeutics with customized functions.

Pan-phylum Comparison of Nematode Metabolic Potential

Nematodes are among the most important causative pathogens of neglected tropical diseases. The increased availability of genomic and transcriptomic data for many understudied nematode species provides a great opportunity to investigate different aspects of their biology. Increasingly, metabolic potential of pathogens is recognized as a critical determinant governing their development, growth and pathogenicity. Comparing metabolic potential among species with distinct trophic ecologies can provide insights on overall biology or molecular adaptations. Furthermore, ascertaining gene expression at pathway level can help in understanding metabolic dynamics over development. Comparison of biochemical pathways (or subpathways, i.e. pathway modules) among related species can also retrospectively indicate potential mistakes in gene-calling and functional annotation. We show with numerous illustrative case studies that comparisons at the level of pathway modules have the potential to uncover biological insights while remaining computationally tractable. Here, we reconstruct and compare metabolic modules found in the deduced proteomes of 13 nematodes and 10 non-nematode species (including hosts of the parasitic nematode species). We observed that the metabolic potential is, in general, concomitant with phylogenetic and/or ecological similarity. Varied metabolic strategies are required among the nematodes, with only 8 out of 51 pathway modules being completely conserved. Enzyme comparison based on topology of metabolic modules uncovered diversification between parasite and host that can potentially guide therapeutic intervention. Gene expression data from 4 nematode species were used to study metabolic dynamics over their life cycles. We report unexpected differential metabolism between immature and mature microfilariae of the human filarial parasite Brugia malayi. A set of genes potentially important for parasitism is also reported, based on an analysis of gene expression in C. elegans and the human hookworm Necator americanus. We illustrate how analyzing and comparing metabolism at the level of pathway modules can improve existing knowledge of nematode metabolic potential and can provide parasitism related insights. Our reconstruction and comparison of nematode metabolic pathways at a pan-phylum and inter-phylum level enabled determination of phylogenetic restrictions and differential expression of pathways. A visualization of our results is available at http://nematode.net and the program for identification of module completeness (modDFS) is freely available at SourceForge. The methods reported will help biologists to predict biochemical potential of any organism with available deduced proteome, to direct experiments and test hypotheses.

We reconstructed metabolic pathways of 23 organisms including 13 nematode species, using their complete deduced protein coding sequences and compared them to 10 non-nematodes. We observed that metabolic potential availability is concomitant with phylogenetic and/or ecological similarity, with the exceptions providing interesting case studies. We also studied changes in metabolic profiles under different developmental stages of 4 nematode species using stage-specific transcriptomic data. A comparison of the variation patterns in these profiles led to recognition of modules that share metabolic profiles at various life-cycle stages or during development. The undertaken analysis improved genome annotation and the obtained results provided insight into parasitism, resulting in identification of taxonomically-restricted pathways and enzymes that may provide new mechanisms for control of nematode infections.

Subcellular Targeting of Proteins Involved in Modification of Plant N- and O-Glycosylation

Plants are attractive expression hosts for the production of recombinant glycoprotein therapeutics. The quality and efficiency of these biopharmaceuticals are very often influenced by the glycosylation profile. Consequently, approaches are needed that enable the production of recombinant glycoproteins with customized and homogenous N- and O-glycan structures. Here, we describe convenient tools that allow targeting and retention of glycan-modifying enzymes in the early secretory pathway of plants. These protocols can be used to fine-tune the subcellular localization of glycosyltransferases and glycosidases in plants and consequently to increase the homogeneity of glycosylation on recombinant glycoproteins.

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.

Transient Expression of Secretory IgA In Planta is Optimal Using a Multi-Gene Vector and may be Further Enhanced by Improving Joining Chain Incorporation

Secretory IgA (sIgA) is a crucial antibody in host defense at mucosal surfaces. It is a promising antibody isotype in a variety of therapeutic settings such as passive vaccination and treatment of inflammatory disorders. However, heterologous production of this heteromultimeric protein complex is still suboptimal. The challenge is the coordinate expression of the four required polypeptides; the alpha heavy chain, the light chain, the joining chain, and part of the polymeric-Ig-receptor called the secretory component, in a 4:4:1:1 ratio. We evaluated the transient expression of three sIgAκ variants, harboring the heavy chain isotype α1, α2m1, or α2m2, of the clinical antibody Ustekinumab in planta. Ustekinumab is directed against the p40 subunit that is shared by the pro-inflammatory cytokines interleukin (IL)-12 and IL-23. A sIgA variant of this antibody may enable localized treatment of inflammatory bowel disease. Of the three different sIgA variants we obtained the highest yield with sIgA1κ reaching up to 373 μg sIgA/mg total soluble protein. The use of a multi-cassette vector containing all four expression cassettes was most efficient. However, not the expression strategy, but the incorporation of the joining chain turned out to be the limiting step for sIgA production. Our data demonstrate that transient expression in planta is suitable for the economic production of heteromultimeric protein complexes such as sIgA.

Plant-based solutions for veterinary immunotherapeutics and prophylactics

An alarming increase in emergence of antibiotic resistance among pathogens worldwide has become a serious threat to our ability to treat infectious diseases according to the World Health Organization. Extensive use of antibiotics by livestock producers promotes the spread of new resistant strains, some of zoonotic concern, which increases food-borne illness in humans and causes significant economic burden on healthcare systems. Furthermore, consumer preferences for meat/poultry/fish produced without the use of antibiotics shape today's market demand. So, it is viewed as inevitable by the One Health Initiative that humans need to reduce the use of antibiotics and turn to alternative, improved means to control disease: vaccination and prophylactics. Besides the intense research focused on novel therapeutic molecules, both these strategies rely heavily on the availability of cost-effective, efficient and scalable production platforms which will allow large-volume manufacturing for vaccines, antibodies and other biopharmaceuticals. Within this context, plant-based platforms for production of recombinant therapeutic proteins offer significant advantages over conventional expression systems, including lack of animal pathogens, low production costs, fast turnaround and response times and rapid, nearly-unlimited scalability. Also, because dried leaves and seeds can be stored at room temperature for lengthy periods without loss of recombinant proteins, plant expression systems have the potential to offer lucrative benefits from the development of edible vaccines and prophylactics, as these would not require "cold chain" storage and transportation, and could be administered in mass volumes with minimal processing. Several biotechnology companies currently have developed and adopted plant-based platforms for commercial production of recombinant protein therapeutics. In this manuscript, we outline the challenges in the process of livestock immunization as well as the current plant biotechnology developments aimed to address these challenges.

Noctilisin, a Venom Glycopeptide of Sirex noctilio (Hymenoptera: Siricidae), Causes Needle Wilt and Defense Gene Responses in Pines

During oviposition, female Sirex noctilio (F.) (Siricidae) woodwasps inject their conifer hosts with a venom gland secretion. The secretion induces a variety of host physiological changes that facilitate subsequent lethal infection by a symbiotic fungus. A heat-stable factor that can migrate from the site of oviposition in the trunk through the xylem to needles in the crown of attacked pines was purified by size-fractionation and reversed-phase-high-performance liquid chromatography using activity assays based on defense gene induction as well as the needle wilt response in pine shoot explants. An 11-amino acid, posttranslationally modified peptide (SEGPROGTKRP) encoded by the most abundant transcript recovered from S. noctilio venom gland tissue comprised the backbone of the 1,850 Da active factor. Posttranslational modifications included hydroxylation of a Pro residue at position 6 as well as O-glycosylation of Ser and Thr residues at positions 1 and 8, respectively. The O-linked sugars were identical α-linked N-acetylgalactosamine residues modified at the C6 position by addition of phosphoethanolamine. In contrast to the native peptide, a synthetic version of the hydroxylated peptide backbone lacking the glycosyl side chains failed to induce pine defense genes or cause needle wilt in excised shoots. This peptide, hereafter called noctilisin, is related to the O-glycosylated short-chain proline-rich antimicrobial peptides exemplified by drosocin. The noctilisin structure contains motifs which may explain how it avoids detection by pine defense systems.

Plant molecular pharming for the treatment of chronic and infectious diseases

Plant molecular pharming has emerged as a niche technology for the manufacture of pharmaceutical products indicated for chronic and infectious diseases, particularly for products that do not fit into the current industry-favored model of fermenter-based production campaigns. In this review, we explore the areas where molecular pharming can make the greatest impact, including the production of pharmaceuticals that have novel glycan structures or that cannot be produced efficiently in microbes or mammalian cells because they are insoluble or toxic. We also explore the market dynamics that encourage the use of molecular pharming, particularly for pharmaceuticals that are required in small amounts (such as personalized medicines) or large amounts (on a multi-ton scale, such as blood products and microbicides) and those that are needed in response to emergency situations (pandemics and bioterrorism). The impact of molecular pharming will increase as the platforms become standardized and optimized through adoption of good manufacturing practice (GMP) standards for clinical development, offering a new opportunity to produce inexpensive medicines in regional markets that are typically excluded under current business models.

A gene responsible for prolyl-hydroxylation of moss-produced recombinant human erythropoietin

Recombinant production of pharmaceutical proteins is crucial, not only for personalized medicine. While most biopharmaceuticals are currently produced in mammalian cell culture, plant-made pharmaceuticals gain momentum. Post-translational modifications in plants are similar to those in humans, however, existing differences may affect quality, safety and efficacy of the products. A frequent modification in higher eukaryotes is prolyl-4-hydroxylase (P4H)-catalysed prolyl-hydroxylation. P4H sequence recognition sites on target proteins differ between humans and plants leading to non-human posttranslational modifications of recombinant human proteins produced in plants. The resulting hydroxyprolines display the anchor for plant-specific O-glycosylation, which bears immunogenic potential for patients. Here we describe the identification of a plant gene responsible for non-human prolyl-hydroxylation of human erythropoietin (hEPO) recombinantly produced in plant (moss) bioreactors. Targeted ablation of this gene abolished undesired prolyl-hydroxylation of hEPO and thus paves the way for plant-made pharmaceuticals humanized via glyco-engineering in moss bioreactors.

Expression of recombinant antibodies

Recombinant antibodies are highly specific detection probes in research, diagnostics, and have emerged over the last two decades as the fastest growing class of therapeutic proteins. Antibody generation has been dramatically accelerated by in vitro selection systems, particularly phage display. An increasing variety of recombinant production systems have been developed, ranging from Gram-negative and positive bacteria, yeasts and filamentous fungi, insect cell lines, mammalian cells to transgenic plants and animals. Currently, almost all therapeutic antibodies are still produced in mammalian cell lines in order to reduce the risk of immunogenicity due to altered, non-human glycosylation patterns. However, recent developments of glycosylation-engineered yeast, insect cell lines, and transgenic plants are promising to obtain antibodies with "human-like" post-translational modifications. Furthermore, smaller antibody fragments including bispecific antibodies without any glycosylation are successfully produced in bacteria and have advanced to clinical testing. The first therapeutic antibody products from a non-mammalian source can be expected in coming next years. In this review, we focus on current antibody production systems including their usability for different applications.

Engineering of human-type O-glycosylation in Nicotiana benthamiana plants

Therapeutic properties of recombinant proteins are very often affected by the composition and heterogeneity of their glycans. Conventional expression systems for recombinant pharmaceutical proteins typically do not address this problem and produce a mixture of glycoforms that are neither identical to human glycans nor optimized for enhanced efficacy. In terms of glycosylation, plants offer certain advantages over mammalian cells as the N-glycosylation pathway of plants is comparably simple and a typical mammalian O-glycosylation pathway is not present at all. During the last ten years we have developed a plant-based expression platform for the generation of recombinant glycoproteins with defined N-glycans. Now we have extended our tool-box for glyco-engineering in the tobacco related species Nicotiana benthamiana toward the production of tailored mucin-type O-glycans on recombinant proteins.

Functional identification of a hydroxyproline-o-galactosyltransferase specific for arabinogalactan protein biosynthesis in Arabidopsis

Although plants contain substantial amounts of arabinogalactan proteins (AGPs), the enzymes responsible for AGP glycosylation are largely unknown. Bioinformatics indicated that AGP galactosyltransferases (GALTs) are members of the carbohydrate-active enzyme glycosyltransferase (GT) 31 family (CAZy GT31) involved in N- and O-glycosylation. Six Arabidopsis GT31 members were expressed in Pichia pastoris and tested for enzyme activity. The At4g21060 gene (named AtGALT2) was found to encode activity for adding galactose (Gal) to hydroxyproline (Hyp) in AGP protein backbones. AtGALT2 specifically catalyzed incorporation of [(14)C]Gal from UDP-[(14)C]Gal to Hyp of model substrate acceptors having AGP peptide sequences, consisting of non-contiguous Hyp residues, such as (Ala-Hyp) repetitive units exemplified by chemically synthesized (AO)7 and anhydrous hydrogen fluoride-deglycosylated d(AO)51. Microsomal preparations from Pichia cells expressing AtGALT2 incorporated [(14)C]Gal to (AO)7, and the resulting product co-eluted with (AO)7 by reverse-phase HPLC. Acid hydrolysis of the [(14)C]Gal-(AO)7 product released (14)C-radiolabel as Gal only. Base hydrolysis of the [(14)C]Gal-(AO)7 product released a (14)C-radiolabeled fragment that co-eluted with a Hyp-Gal standard after high performance anion-exchange chromatography fractionation. AtGALT2 is specific for AGPs because substrates lacking AGP peptide sequences did not act as acceptors. Moreover, AtGALT2 uses only UDP-Gal as the substrate donor and requires Mg(2+) or Mn(2+) for high activity. Additional support that AtGALT2 encodes an AGP GALT was provided by two allelic AtGALT2 knock-out mutants, which demonstrated lower GALT activities and reductions in β-Yariv-precipitated AGPs compared with wild type plants. Confocal microscopic analysis of fluorescently tagged AtGALT2 in tobacco epidermal cells indicated that AtGALT2 is probably localized in the endomembrane system consistent with its function.

Engineering of sialylated mucin-type O-glycosylation in plants

Proper N- and O-glycosylation of recombinant proteins is important for their biological function. Although the N-glycan processing pathway of different expression hosts has been successfully modified in the past, comparatively little attention has been paid to the generation of customized O-linked glycans. Plants are attractive hosts for engineering of O-glycosylation steps, as they contain no endogenous glycosyltransferases that perform mammalian-type Ser/Thr glycosylation and could interfere with the production of defined O-glycans. Here, we produced mucin-type O-GalNAc and core 1 O-linked glycan structures on recombinant human erythropoietin fused to an IgG heavy chain fragment (EPO-Fc) by transient expression in Nicotiana benthamiana plants. Furthermore, for the generation of sialylated core 1 structures constructs encoding human polypeptide:N-acetylgalactosaminyltransferase 2, Drosophila melanogaster core 1 β1,3-galactosyltransferase, human α2,3-sialyltransferase, and Mus musculus α2,6-sialyltransferase were transiently co-expressed in N. benthamiana together with EPO-Fc and the machinery for sialylation of N-glycans. The formation of significant amounts of mono- and disialylated O-linked glycans was confirmed by liquid chromatography-electrospray ionization-mass spectrometry. Analysis of the three EPO glycopeptides carrying N-glycans revealed the presence of biantennary structures with terminal sialic acid residues. Our data demonstrate that N. benthamiana plants are amenable to engineering of the O-glycosylation pathway and can produce well defined human-type O- and N-linked glycans on recombinant therapeutics.

Toward stable genetic engineering of human O-glycosylation in plants

Glycosylation is the most abundant and complex posttranslational modification to be considered for recombinant production of therapeutic proteins. Mucin-type (N-acetylgalactosamine [GalNAc]-type) O-glycosylation is found in eumetazoan cells but absent in plants and yeast, making these cell types an obvious choice for de novo engineering of this O-glycosylation pathway. We previously showed that transient implementation of O-glycosylation capacity in plants requires introduction of the synthesis of the donor substrate UDP-GalNAc and one or more polypeptide GalNAc-transferases for incorporating GalNAc residues into proteins. Here, we have stably engineered O-glycosylation capacity in two plant cell systems, soil-grown Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum) Bright Yellow-2 suspension culture cells. Efficient GalNAc O-glycosylation of two stably coexpressed substrate O-glycoproteins was obtained, but a high degree of proline hydroxylation and hydroxyproline-linked arabinosides, on a mucin (MUC1)-derived substrate, was also observed. Addition of the prolyl 4-hydroxylase inhibitor 2,2-dipyridyl, however, effectively suppressed proline hydroxylation and arabinosylation of MUC1 in Bright Yellow-2 cells. In summary, stably engineered mammalian type O-glycosylation was established in transgenic plants, demonstrating that plants may serve as host cells for the production of recombinant O-glycoproteins. However, the present stable implementation further strengthens the notion that elimination of endogenous posttranslational modifications may be needed for the production of protein therapeutics.

Carbohydrate clearance receptors in transfusion medicine

BACKGROUND

Complex carbohydrates play important functions for circulation of proteins and cells. They provide protective shields and refraction from non-specific interactions with negative charges from sialic acids to enhance circulatory half-life. For recombinant protein therapeutics carbohydrates are especially important to enhance size and reduce glomerular filtration loss. Carbohydrates are, however, also ligands for a large number of carbohydrate-binding lectins exposed to the circulatory system that serve as scavenger receptors for the innate immune system, or have more specific roles in targeting of glycoproteins and cells.

SCOPE OF REVIEW

Here we provide an overview of the common lectin receptors that play roles for circulating glycoproteins and cells, and present a discussion of ways to engineer glycosylation of recombinant biologics and cells to improve therapeutic effects.

MAJOR CONCLUSIONS

While the pharmaceutical industry has learned how to exploit carbohydrates to improve pharmacokinetic properties of recombinant therapeutics, our understanding of how to improve cell-based therapies by manipulation of complex carbohydrates is still at its infancy. Progress with the latter has recently been achieved with cold-stored platelets, where exposure of uncapped glycans lead to rapid clearance from circulation by several lectin-mediated pathways.

GENERAL SIGNIFICANCE

Understanding lectin-mediated clearance pathways is essential for progress in development of biological pharmaceuticals.

Analytical technologies for identification and characterization of the plant N-glycoproteome

N-glycosylation is one of the most common and complex post-translational modifications of eukaryotic proteins and one that has numerous roles, such as modulating protein stability, sorting, folding, enzyme activity, and ligand interactions. In plants, the functional significance of N-glycosylation is typically obscure, although it is a feature of most secreted proteins and so is potentially of considerable interest to plant cell wall biologists. While analytical pipelines have been established to characterize yeast, mammalian, and bacterial N-glycoproteomes, such large-scale approaches for the study of plant glycoproteins have yet to be reported. Indeed, the N-glycans that decorate plant and mammalian or yeast proteins are structurally distinct and so modification of existing analytical approaches are needed to tackle plant N-glycoproteomes. In this review, we summarize a range of existing technologies for large-scale N-glycoprotein analysis and highlight promising future approaches that may provide a better understanding of the plant N-glycoproteome, and therefore the cell wall proteome and other proteins associated with the secretory pathway.

Challenges in O-glycan engineering of plants

Plants are attractive alternative expression hosts for the production of recombinant proteins. Many therapeutic proteins are glycosylated with N- and O-glycosylation being the most prevalent forms of protein glycosylation. While N-glycans have already been modified in plants toward the formation of homogenous mammalian-type glycoforms with equal or improved biological function compared to mammalian-cell culture produced glycoproteins little attention has been paid to the modification of O-linked glycans. Recently, the first step of mammalian O-glycan biosynthesis has been accomplished in plants. However, as outlined in this short review there are important issues that have to be addressed in the future. These include: (i) elimination of potentially immunogenic or allergenic carbohydrate epitopes containing arabinosides or arabinogalactans, (ii) a detailed investigation of the interplay between engineered N- and O-glycosylation pathways to avoid competition for common metabolites like UDP-GlcNAc, and (iii) a deeper understanding of signals and mechanisms for distribution of glycan processing enzymes, which is a prerequisite for complete and homogenous glycosylation of recombinant proteins.