Biologically active, magnICON®-expressed EPO-Fc from stably transformed Nicotiana benthamiana plants presenting tetra-antennary N-glycan structures

Impact of glycoengineering and antidrug antibodies on the anticancer activity of a plant-made lectin-Fc fusion protein

Plants are an efficient production platform for manufacturing glycoengineered monoclonal antibodies and antibody-like molecules. Avaren-Fc (AvFc) is a lectin-Fc fusion protein or lectibody produced in Nicotiana benthamiana, which selectively recognizes cancer-associated high-mannose glycans. In this study, we report the generation of a glycovariant of AvFc that is devoid of plant glycans, including the core α1,3-fucose and β1,2-xylose residues. The successful removal of these glycans was confirmed by glycan analysis using HPLC. This variant, AvFc^ΔXF , has significantly higher affinity for Fc gamma receptors and induces higher levels of luciferase expression in an antibody-dependent cell-mediated cytotoxicity (ADCC) reporter assay against B16F10 murine melanoma cells without inducing apoptosis or inhibiting proliferation. In the B16F10 flank tumour mouse model, we found that systemic administration of AvFc^ΔXF , but not an aglycosylated AvFc variant lacking affinity for Fc receptors, significantly delayed the growth of tumours, suggesting that Fc-mediated effector functions were integral. AvFc^ΔXF treatment also significantly reduced lung metastasis of B16F10 upon intravenous challenge whereas a sugar-binding-deficient mutant failed to show efficacy. Lastly, we determined the impact of antidrug antibodies (ADAs) on drug activity in vivo by pretreating animals with AvFc^ΔXF before implanting tumours. Despite a significant ADA response induced by the pretreatment, we found that the activity of AvFc^ΔXF was unaffected by the presence of these antibodies. These results demonstrate that glycoengineering is a powerful strategy to enhance AvFc's antitumor activity.

Production of the SARS-CoV-2 Spike protein and its Receptor Binding Domain in plant cell suspension cultures

The COVID-19 pandemic, caused by the worldwide spread of SARS-CoV-2, has prompted the scientific community to rapidly develop efficient and specific diagnostics and therapeutics. A number of avenues have been explored, including the manufacture of COVID-related proteins to be used as reagents for diagnostics or treatment. The production of RBD and Spike proteins was previously achieved in eukaryotic cells, mainly mammalian cell cultures, while the production in microbial systems has been unsuccessful until now. Here we report the effective production of SARS-CoV-2 proteins in two plant model systems. We established transgenic tobacco BY-2 and Medicago truncatula A17 cell suspension cultures stably producing the full-length Spike and RBD recombinant proteins. For both proteins, various glycoforms were obtained, with higher yields in Medicago cultures than BY-2. This work highlights that RBD and Spike can be secreted into the culture medium, which will impact subsequent purification and downstream processing costs. Analysis of the culture media indicated the presence of the high molecular weight Spike protein of SARS-CoV-2. Although the production yields still need improvement to compete with mammalian systems, this is the first report showing that plant cell suspension cultures are able to produce the high molecular weight Spike protein. This finding strengthens the potential of plant cell cultures as production platforms for large complex proteins.

High yield secretion of human erythropoietin from tobacco cells for ex vivo differentiation of hematopoietic stem cells towards red blood cells

Human erythropoietin (EPO) is a key cytokine in erythropoiesis by regulating differentiation of erythroid progenitor cells into red blood cells (RBCs). Plant cell cultures are considered as promising alternative bioproduction platforms for EPO. To overcome the bottlenecks of low protein productivity and secretion, EPO was expressed in tobacco BY-2 cells with a designer peptide tag, termed (SP)20 that consists of 20 tandem repeats of a "Ser-Pro" motif. This de novo designed tag directed extensive O-glycosylation on each Pro residue in plant cells and acted as a molecular carrier to promote the extracellular secretion of EPO. To facilitate the establishment of stable and high-expression BY-2 cell lines, EPO molecules were co-expressed with a reporter protein GFP, which could be used as a visual marker to monitor the protein expression during the subculture. The engineered (SP)20 glycomodule substantially increased the secreted yields of EPO up to 4.31 μg/mL. The (SP)20-tagged EPOs exhibited the expected activity in promoting the proliferation of TF-1 cells, though their EC50 was 12-fold higher than that of EPO standard. The (SP)20-tagged EPOs could also stimulate the ex vivo expansion and differentiation of hematopoietic stem cell (CD34+ cells) towards RBCs.

The Instability of Dimeric Fc-Fusions Expressed in Plants Can Be Solved by Monomeric Fc Technology

The potential therapeutic value of many proteins is ultimately limited by their rapid in vivo clearance. One strategy to limit clearance by metabolism and excretion, and improving the stability of therapeutic proteins, is their fusion to the immunoglobulin fragment crystallizable region (Fc). The Fc region plays multiple roles in (i) dimerization for the formation of "Y"-shaped structure of Ig, (ii) Fc-mediated effector functions, (iii) extension of serum half-life, and (iv) a cost-effective purification tag. Plants and in particular Nicotiana benthamiana have proven to be suitable expression platforms for several recombinant therapeutic proteins. Despite the enormous success of their use for the production of full-length monoclonal antibodies, the expression of Fc-fused therapeutic proteins in plants has shown limitations. Many Fc-fusion proteins expressed in plants show different degrees of instability resulting in high amounts of Fc-derived degradation products. To address this issue, we used erythropoietin (EPO) as a reporter protein and evaluated the efforts to enhance the expression of full-length EPO-Fc targeted to the apoplast of N. benthamiana. Our results show that the instability of the fusion protein is independent from the Fc origin or IgG subclass and from the peptide sequence used to link the two domains. We also show that a similar instability occurs upon the expression of individual heavy chains of monoclonal antibodies and ScFv-Fc that mimic the "Y"-shape of antibodies but lack the light chain. We propose that in this configuration, steric hindrance between the protein domains leads to physical instability. Indeed, mutations of critical residues located on the Fc dimerization interface allowed the expression of fully stable EPO monomeric Fc-fusion proteins. We discuss the limitations of Fc-fusion technology in N. benthamiana transient expression systems and suggest strategies to optimize the Fc-based scaffolds on their folding and aggregation resistance in order to improve the stability.

The N-Glycosylation Processing Potential of the Mammalian Golgi Apparatus

Heterogeneity is an inherent feature of the glycosylation process. Mammalian cells often produce a variety of glycan structures on separate molecules of the same protein, known as glycoforms. This heterogeneity is not random but is controlled by the organization of the glycosylation machinery in the Golgi cisternae. In this work, we use a computational model of the N-glycosylation process to probe how the organization of the glycosylation machinery into different cisternae drives N-glycan biosynthesis toward differing degrees of heterogeneity. Using this model, we demonstrate the N-glycosylation potential and limits of the mammalian Golgi apparatus, for example how the number of cisternae limits the goal of achieving near homogeneity for N-glycans. The production of specific glycoforms guided by this computational study could pave the way for “glycoform engineering,” which will find uses in the functional investigation of glycans, the modulation of glycan-mediated physiological functions, and in biotechnology.

N-glycan maturation mutants in Lotus japonicus for basic and applied glycoprotein research

Studies of protein N-glycosylation are important for answering fundamental questions on the diverse functions of glycoproteins in plant growth and development. Here we generated and characterised a comprehensive collection of Lotus japonicusLORE1 insertion mutants, each lacking the activity of one of the 12 enzymes required for normal N-glycan maturation in the glycosylation machinery. The inactivation of the individual genes resulted in altered N-glycan patterns as documented using mass spectrometry and glycan-recognising antibodies, indicating successful identification of null mutations in the target glyco-genes. For example, both mass spectrometry and immunoblotting experiments suggest that proteins derived from the α1,3-fucosyltransferase (Lj3fuct) mutant completely lacked α1,3-core fucosylation. Mass spectrometry also suggested that the Lotus japonicus convicilin 2 was one of the main glycoproteins undergoing differential expression/N-glycosylation in the mutants. Demonstrating the functional importance of glycosylation, reduced growth and seed production phenotypes were observed for the mutant plants lacking functional mannosidase I, N-acetylglucosaminyltransferase I, and α1,3-fucosyltransferase, even though the relative protein composition and abundance appeared unaffected. The strength of our N-glycosylation mutant platform is the broad spectrum of resulting glycoprotein profiles and altered physiological phenotypes that can be produced from single, double, triple and quadruple mutants. This platform will serve as a valuable tool for elucidating the functional role of protein N-glycosylation in plants. Furthermore, this technology can be used to generate stable plant mutant lines for biopharmaceutical production of glycoproteins displaying relative homogeneous and mammalian-like N-glycosylation features.

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.

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

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

Controlled glycosylation of plant-produced recombinant proteins

Despite their recognized importance for therapeutic proteins, the production of structurally defined glycans is still a challenging issue. However, an increased understanding of glycosylation pathways, recent advances in analytical tools, and emerging technologies for subcellular targeting using chimeric glycosyltransferases are facilitating the rational design of new glycan biosynthetic pathways. Plants are particularly amenable to glyco-engineering approaches and thus they are increasingly being used for the production of recombinant proteins. Here we summarize the main achievements in the field of in planta glyco-engineering for the production of therapeutically relevant proteins.

Plant glyco-biotechnology on the way to synthetic biology

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

Fusion of an Fc chain to a VHH boosts the accumulation levels in Arabidopsis seeds

Nanobodies® (VHHs) provide powerful tools in therapeutic and biotechnological applications. Nevertheless, for some applications, bivalent antibodies perform much better, and for this, an Fc chain can be fused to the VHH domain, resulting in a bivalent homodimeric VHH-Fc complex. However, the production of bivalent antibodies in Escherichia coli is rather inefficient. Therefore, we compared the production of VHH7 and VHH7-Fc as antibodies of interest in Arabidopsis seeds for detecting prostate-specific antigen (PSA), a well-known biomarker for prostate cancer in the early stages of tumour development. The influence of the signal sequence (camel versus plant) and that of the Fc chain origin (human, mouse or pig) were evaluated. The accumulation levels of VHHs were very low, with a maximum of 0.13% VHH of total soluble protein (TSP) in homozygous T3 seeds, while VHH-Fc accumulation levels were at least 10- to 100-fold higher, with a maximum of 16.25% VHH-Fc of TSP. Both the camel and plant signal peptides were efficiently cleaved off and did not affect the accumulation levels. However, the Fc chain origin strongly affected the degree of proteolysis, but only had a slight influence on the accumulation level. Analysis of the mRNA levels suggested that the low amount of VHHs produced in Arabidopsis seeds was not due to a failure in transcription, but rather to translation inefficiency, protein instability and/or degradation. Most importantly, the plant-produced VHH7 and VHH7-Fc antibodies were functional in detecting PSA and could thus be used for diagnostic applications.

Expression of functionally active sialylated human erythropoietin in plants

Recombinant human erythropoietin (rhEPO), a glycohormone, is one of the leading biopharmaceutical products. The production of rhEPO is currently restricted to mammalian cell expression systems because of rhEPO's highly complex glycosylation pattern, which is a major determinant for drug-efficacy. Here we evaluate the ability of plants to produce different glycoforms of rhEPO. cDNA constructs were delivered to Nicotiana benthamiana (N. benthamiana) and transiently expressed by a viral based expression system. Expression levels up to 85 mg rhEPO/kg fresh leaf material were achieved. Moreover, co-expression of rhEPO with six mammalian genes required for in planta protein sialylation resulted in the synthesis of rhEPO decorated mainly with bisialylated N-glycans (NaNa), the most abundant glycoform of circulating hEPO in patients with anemia. A newly established peptide tag (ELDKWA) fused to hEPO was particularly well-suited for purification of the recombinant hormone based on immunoaffinity. Subsequent lectin chromatography allowed enrichment of exclusively sialylated rhEPO. All plant-derived glycoforms exhibited high biological activity as determined by a cell-based receptor-binding assay. The generation of rhEPO carrying largely homogeneous glycosylation profiles (GnGnXF, GnGn, and NaNa) will facilitate further investigation of functionalities with potential implications for medical 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.

Generation of biologically active multi-sialylated recombinant human EPOFc in plants

Hyperglycosylated proteins are more stable, show increased serum half-life and less sensitivity to proteolysis compared to non-sialylated forms. This applies particularly to recombinant human erythropoietin (rhEPO). Recent progress in N-glycoengineering of non-mammalian expression hosts resulted in in vivo protein sialylation at great homogeneity. However the synthesis of multi-sialylated N-glycans is so far restricted to mammalian cells. Here we used a plant based expression system to accomplish multi-antennary protein sialylation. A human erythropoietin fusion protein (EPOFc) was transiently expressed in Nicotiana benthamiana ΔXTFT, a glycosylation mutant that lacks plant specific N-glycan residues. cDNA of the hormone was co-delivered into plants with the necessary genes for (i) branching (ii) β1,4-galactosylation as well as for the (iii) synthesis, transport and transfer of sialic acid. This resulted in the production of recombinant EPOFc carrying bi- tri- and tetra-sialylated complex N-glycans. The formation of this highly complex oligosaccharide structure required the coordinated expression of 11 human proteins acting in different subcellular compartments at different stages of the glycosylation pathway. In vitro receptor binding assays demonstrate the generation of biologically active molecules. We demonstrate the in planta synthesis of one of the most complex mammalian glycoforms pointing to an outstanding high degree of tolerance to changes in the glycosylation pathway in plants.

Human granulocyte colony-stimulating factor (hG-CSF) expression in plastids of Lactuca sativa

BACKGROUND

Human granulocyte colony-stimulating factor (hG-CSF) can serve as valuable biopharmaceutical for research and treatment of the human blood cancer. Transplastomic plants have been emerged as a new and high potential candidate for production of recombinant biopharmaceutical proteins in comparison with transgenic plants due to extremely high level expression, biosafety and many other advantages.

METHODS

hG-CSF gene was cloned into pCL vector between prrn16S promoter and TpsbA terminator. The recombinant vector was coated on nanogold particles and transformed to lettuce chloroplasts through biolistic method. Callogenesis and regeneration of cotyledonary explants were obtained by Murashige and Skoog media containing 6-benzylaminopurine and 1-naphthaleneacetic acid hormones. The presence of hG-CSF gene in plastome was studied with four specific PCR primers and expression by Western immunoblotting.

RESULTS

hG-CSF gene cloning was confirmed by digestion and sequencing. Transplastomic lettuce lines were regenerated and subjected to molecular analysis. The presence of hG-CSF in plastome was confirmed by PCR using specific primers designed from the plastid genome. Western immunoblotting of extracted protein from transplastomic plants showed a 20-kDa band, which verified the expression of recombinant protein in lettuce chloroplasts.

CONCLUSIONS

This study is the first report that successfully express hG-CSF gene in lettuce chloroplast. The lettuce plastome can provide a cheap and safe expression platform for producing valuable biopharmaceuticals for research and treatment.

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.