Non-food/feed seeds as biofactories for the high-yield production of recombinant pharmaceuticals

Molecular farming for sustainable production of clinical-grade antimicrobial peptides

Antimicrobial peptides (AMPs) are emerging as next-generation therapeutics due to their broad-spectrum activity against drug-resistant bacterial strains and their ability to eradicate biofilms, modulate immune responses, exert anti-inflammatory effects and improve disease management. They are produced through solid-phase peptide synthesis or in bacterial or yeast cells. Molecular farming, i.e. the production of biologics in plants, offers a low-cost, non-toxic, scalable and simple alternative platform to produce AMPs at a sustainable cost. In this review, we discuss the advantages of molecular farming for producing clinical-grade AMPs, advances in expression and purification systems and the cost advantage for industrial-scale production. We further review how 'green' production is filling the sustainability gap, streamlining patent and regulatory approvals and enabling successful clinical translations that demonstrate the future potential of AMPs produced by molecular farming. Finally, we discuss the regulatory challenges that need to be addressed to fully realize the potential of molecular farming-based AMP production for therapeutics.

Influence on Accumulation Levels and Subcellular Localization of Prolamins by Fusion with the Functional Peptide in Transgenic Rice Seeds

To exploit the rice seed-based oral vaccine against Sjögren's syndrome, altered peptide ligand of N-terminal 1 (N1-APL7) from its M3 muscarinic acetylcholine receptor (M3R) autoantigen was expressed as fusion protein with the representative four types of rice prolamins (16 kDa, 14 kDa, 13 kDa, and 10 kDa prolamins) under the control of the individual native prolamin promoter. The 10kD:N1-APL7 and 14kD:N1-APL7 accumulated at high levels (287 and 58 µg/grain), respectively, whereas production levels of the remaining ones were remarkably low. Co-expression of these fusion proteins did not enhance the accumulation level of N1-APL7 in an additive manner. Downregulation of endogenous seed storage proteins by RNAi-mediated suppression also did not lead to substantial elevation of the co-expressed prolamin:N1-APL7 products. When transgenic rice seeds were subjected to in vitro proteolysis with pepsin, the 10kD:N1-APL7 was digested more quickly than the endogenous 10 kDa prolamin and the 14kD:N1-APL7 deposited in PB-Is. This difference could be explained by the finding that the 10kD:N1-APL7 was unexpectedly localized in the PB-IIs containing glutelins. These results indicated that not only accumulation level but also subcellular localization of inherent prolamins were highly influenced by the liked N1-APL7 peptide.

Fusing the 3'UTR of seed storage protein genes leads to massive recombinant protein accumulation in seeds

The demand for recombinant proteins is rising dramatically, and effective production systems are currently being developed. The production of recombinant proteins in plants is a promising approach due to its low cost and low risk of contamination of the proteins with endotoxins or infectious agents from the culture serum. Plant seeds primarily accumulate seed storage proteins (SSPs), which are transcribed and translated from a few genes; therefore, the mechanism underlying SSP accumulation has been studied to help devise ways to increase recombinant protein production. We found that the 3'UTR of SSP genes are essential for SSP accumulation and can be used in the production of recombinant proteins in Arabidopsis. Fusion of the 3'UTR of SSP genes to the 3' ends of DNA sequences encoding recombinant proteins enables massive accumulation of recombinant proteins with enzymatic activity in Arabidopsis seeds. This method is also applicable to the production of human Interferon Lambda-3 (IFN-lambda 3), a candidate biopharmaceutical compound against hepatitis C infection. Considering the low cost and ease of protein production in Arabidopsis, as well as the rapid growth of this plant, our method is useful for large-scale preparation of recombinant proteins for both academic research and biopharmaceutical production.

Plant-based biopharmaceutical engineering

Plants can be engineered to recombinantly produce high-quality proteins such as therapeutic proteins and vaccines, also known as molecular farming. Molecular farming can be established in various settings with minimal cold-chain requirements and could thus ensure rapid and global-scale deployment of biopharmaceuticals, promoting equitable access to pharmaceuticals. State of the art plant-based engineering relies on rationally assembled genetic circuits, engineered to enable the high-throughput and rapid expression of multimeric proteins with complex post-translational modifications. In this Review, we discuss the design of expression hosts and vectors, including Nicotiana benthamiana, viral elements and transient expression vectors, for the production of biopharmaceuticals in plants. We examine engineering of post-translational modifications and highlight the plant-based expression of monoclonal antibodies and nanoparticles, such as virus-like particles and protein bodies. Techno-economic analyses suggest a cost advantage of molecular farming compared with mammalian cell-based protein production systems. However, regulatory challenges remain to be addressed to enable the widespread translation of plant-based biopharmaceuticals.

Cell Biology Methods to Study Recombinant Proteins in Seeds

Seeds are an attractive platform for the production of recombinant proteins because of their excellent storage properties and their well-developed endomembrane system, which allows accumulation of the product within specialized storage organelles. Due to the presence of these additional organelles and the resulting complexity of intracellular protein trafficking it is interesting to investigate the transport and storage of a recombinant protein within seed tissues, its interactions with endogenous reserve proteins and its impact on the ultrastructure of the endomembrane system. Possible approaches include sequential extraction procedures, subcellular fractionation and 2D as well as 3D electron microscopy techniques such as electron tomography (ET) and serial block face scanning electron microscopy (SBF-SEM), which are described and discussed in this chapter.

Plant-Based Systems for Vaccine Production

Plant systems have been used as biofactories to produce recombinant proteins since 1983. The huge amount of data, collected so far in this framework, suggests that plants display several key advantages over existing traditional platforms when they are intended for therapeutic uses, including safety, scalability, and the speed in obtaining the final product.Here, we describe a method that could be applied for the expression and production of a candidate subunit vaccine in Nicotiana benthamiana plants by transient expression, defining all the protocols starting from plant cultivation to target recombinant protein purification.

Elastin-like polypeptide and γ-zein fusions significantly increase recombinant protein accumulation in soybean seeds

Soybean seeds are an ideal host for the production of recombinant proteins because of their high content of proteins, long-term stability of seed proteins under ambient conditions, and easy establishment of efficient purification protocols. In this study, a polypeptide fusion strategy was applied to explore the capacity of elastin-like polypeptide (ELP) and γ-zein fusions in increasing the accumulation of the recombinant protein in soybean seeds. Transgenic soybean plants were generated to express the γ-zein- or ELP-fused green fluorescent protein (GFP) under the control of the soybean seed-specific promoter of β-conglycinin alpha subunit (BCSP). Significant differences were observed in the accumulation of zein-GFP and GFP-ELP from that of the unfused GFP in transgenic soybean seeds based on the total soluble protein (TSP), despite the low-copy of T-DNA insertions and similar expression at the mRNA levels in selected transgenic lines. The average levels of zein-GFP and GFP-ELP accumulated in immature seeds of these transgenic lines were 0.99% and 0.29% TSP, respectively, compared with 0.07% TSP of the unfused GFP. In mature soybean seeds, the accumulation of zein-GFP and GFP-ELP proteins was 1.8% and 0.84% TSP, an increase of 3.91- and 1.82-fold, respectively, in comparison with that of the unfused GFP (0.46% TSP). Confocal laser scanning analysis showed that both zein-GFP and GFP-ELP were abundantly deposited in many small spherical particles of transgenic seeds, while there were fewer such florescence signals in the same cellular compartments of the unfused GFP-expressing seeds. Despite increased recombinant protein accumulation, there were no significant changes in the total protein and oil content in seeds between the transgenic and non-transformed plants, suggesting the possible presence of threshold limits of total protein accumulation in transgenic soybean seeds. Overall, our results indicate that γ-zein and ELP fusions significantly increased the accumulation of the recombinant protein, but exhibited no significant influence on the total protein and oil content in soybean seeds.

Efficient Plant Production of Recombinant NS1 Protein for Diagnosis of Dengue

Dengue fever is endemic in more than 120 countries, which account for 3.9 billion people at risk of infection worldwide. The absence of a vaccine with effective protection against the four serotypes of this virus makes differential molecular diagnosis the key step for the correct treatment of the disease. Rapid and efficient diagnosis prevents progression to a more severe stage of this disease. Currently, the limiting factor in the manufacture of dengue (DENV) diagnostic kits is the lack of large-scale production of the non-structural 1 (NS1) protein (antigen) to be used in the capture of antibodies from the blood serum of infected patients. In this work, we use plant biotechnology and genetic engineering as tools for the study of protein production for research and commercial purposes. Gene transfer, integration and expression in plants is a valid strategy for obtaining large-scale and low-cost heterologous protein production. The authors produced NS1 protein of the dengue virus serotype 2 (NS1DENV2) in the Arabidopsis thaliana plant. Transgenic plants obtained by genetic transformation expressed the recombinant protein that was purified and characterized for diagnostic use. The yield was 203 μg of the recombinant protein per gram of fresh leaf. By in situ immunolocalization, transgenic protein was observed within the plant tissue, located in aggregates bodies. These antigens showed high sensitivity and specificity to both IgM (84.29% and 91.43%, respectively) and IgG (83.08% and 87.69%, respectively). The study goes a step further to validate the use of plants as a strategy for obtaining large-scale and efficient protein production to be used in dengue virus diagnostic tests.

Seed-produced anti-globulin VHH-Fc antibodies retrieve globulin precursors in the insoluble fraction and modulate the Arabidopsis thaliana seed subcellular morphology

Nanobody-heavy chain (VHH-Fc) antibody formats have the potential to immunomodulate even highly accumulating proteins and provide a valuable tool to experimentally modulate the subcellular distribution of seed storage proteins. Recombinant antibodies often obtain high accumulation levels in plants, and thus, besides being the actual end-product, antibodies targeting endogenous host proteins can be used to interfere with the localization and functioning of their corresponding antigens. Here, we compared the effect of a seed-expressed nanobody-heavy chain (VHH-Fc) antibody against the highly abundant Arabidopsis thaliana globulin seed storage protein cruciferin with that of a VHH-Fc antibody without endogenous target. Both antibodies reached high accumulation levels of around 10% of total soluble protein, but strikingly, another significant part was present in the insoluble protein fraction and was recovered only after extraction under denaturing conditions. In seeds containing the anti-cruciferin antibodies but not the antibody without endogenous target, the amount of soluble, processed globulin subunits was severely reduced and a major part of the cruciferin molecules was found as precursor in the insoluble fraction. Moreover, in these seeds, aberrant vacuolar phenotypes were observed that were different from the effects caused by the depletion of globulins in knock-out seeds. Remarkably, the seeds with strongly reduced globulin amounts are fully viable and germinate with frequencies similar to wild type, illustrating how flexible seeds can retrieve amino acids from the stored proteins to start germination.

Making plants into cost-effective bioreactors for highly active antimicrobial peptides

As antibiotic-resistant bacterial pathogens become an ever-increasing concern, antimicrobial peptides (AMPs) have grown increasingly attractive as alternatives. Potentially, plants could be used as cost-effective AMP bioreactors; however, reported heterologous AMP expression is much lower in plants than in E. coli expression systems and often results in plant cytotoxicity, even for AMPs fused to carrier proteins. This suggests that there may be a physical characteristic of the previously described heterologous AMPs which impedes efficient expression in plants. Using a meta-analysis of protein databases, this study has determined that native plant AMPs were significantly less cationic than AMPs native to other taxa. To apply this finding to plant expression, the transient expression of 10 different heterologous AMPs, ranging in charge from +7 to -5, was tested in the tobacco, Nicotiana benthamiana. Elastin-like polypeptide (ELP) was used as the carrier protein for AMP expression. ELP fusion allowed for a simple, cost-effective temperature shift purification. Using this system, all five anionic AMPs expressed well, with two at unusually high levels (375 and 563 μg/gfw). Furthermore, antimicrobial activity against Staphylococcus epidermidis was an order of magnitude greater (average minimum inhibitory concentration MIC of 0.26μM) than that typically seen for AMPs expressed in E. coli systems and was associated with the uncleaved fusion peptide. In summary, this study describes a means of expressing AMP fusions in plants in high yield, purified by a simple temperature-shift protocol, resulting in a fusion peptide with high antimicrobial activity and without the need for a peptide cleavage step.

Expression of Single Chain Variable Fragment (scFv) Molecules in Plants: A Comprehensive Update

Single chain variable fragments (scFvs) are generated by joining together the variable heavy and light chain of a monoclonal antibody (mAb) via a peptide linker. They offer some advantages over the parental mAb such as low molecular weight, heterologous production, multimeric form, and multivalency. The scFvs were produced against more than 50 antigens till date using 10 different plant species as the expression system. There were considerable improvements in the expression and purification strategies of scFv in the last 24 years. With the growing demand of scFv in therapeutic and diagnostic fields, its biosynthesis needs to be increased. The easiness in development, maintenance, and multiplication of transgenic plants make them an attractive expression platform for scFv production. The review intends to provide comprehensive information about the use of plant expression system to produce scFv. The developments, advantages, pitfalls, and possible prospects of improvement for the exploitation of plants in the industrial level are discussed.

Transformation strategies for stable expression of complex hetero-multimeric proteins like secretory immunoglobulin A in plants

Plant expression systems have proven to be exceptional in producing high-value complex polymeric proteins such as secretory IgAs (SIgAs). However, polymeric protein production requires the expression of multiple genes, which can be transformed as single or multiple T-DNA units to generate stable transgenic plant lines. Here, we evaluated four strategies to stably transform multiple genes and to obtain high expression of all components. Using the in-seed expression of a simplified secretory IgA (sSIgA) as a reference molecule, we conclude that it is better to spread the genes over two T-DNAs than to contain them in a single T-DNA, because of the presence of homologous recombination events and gene silencing. These T-DNAs can be cotransformed to obtain transgenic plants in one transformation step. However, if time permits, more transformants with high production levels of the polymeric protein can be obtained either by sequential transformation or by in-parallel transformation followed by crossing of transformants independently selected for excellent expression of the genes in each T-DNA.

Russell-Like Bodies in Plant Seeds Share Common Features With Prolamin Bodies and Occur Upon Recombinant Protein Production

Although many recombinant proteins have been produced in seeds at high yields without adverse effects on the plant, endoplasmic reticulum (ER) stress and aberrant localization of endogenous or recombinant proteins have also been reported. The production of murine interleukin-10 (mIL-10) in Arabidopsis thaliana seeds resulted in the de novo formation of ER-derived structures containing a large fraction of the recombinant protein in an insoluble form. These bodies containing mIL-10 were morphologically similar to Russell bodies found in mammalian cells. We confirmed that the compartment containing mIL-10 was enclosed by ER membranes, and 3D electron microscopy revealed that these structures have a spheroidal shape. Another feature shared with Russell bodies is the continued viability of the cells that generate these organelles. To investigate similarities in the formation of Russell-like bodies and the plant-specific protein bodies formed by prolamins in cereal seeds, we crossed plants containing ectopic ER-derived prolamin protein bodies with a line accumulating mIL-10 in Russell-like bodies. This resulted in seeds containing only one population of protein bodies in which mIL-10 inclusions formed a central core surrounded by the prolamin-containing matrix, suggesting that both types of protein aggregates are together removed from the secretory pathway by a common mechanism. We propose that, like mammalian cells, plant cells are able to form Russell-like bodies as a self-protection mechanism, when they are overloaded with a partially transport-incompetent protein, and we discuss the resulting challenges for recombinant protein production.

In planta expression of nanobody-based designer chicken antibodies targeting Campylobacter

Campylobacteriosis is a widespread infectious disease, leading to a major health and economic burden. Chickens are considered as the most common infection source for humans. Campylobacter mainly multiplies in the mucus layer of their caeca. No effective control measures are currently available, but passive immunisation of chickens with pathogen-specific maternal IgY antibodies, present in egg yolk of immunised chickens, reduces Campylobacter colonisation. To explore this strategy further, anti-Campylobacter nanobodies, directed against the flagella and major outer membrane proteins, were fused to the constant domains of chicken IgA and IgY, combining the benefits of nanobodies and the effector functions of the Fc-domains. The designer chimeric antibodies were effectively produced in leaves of Nicotiana benthamiana and seeds of Arabidopsis thaliana. Stable expression of the chimeric antibodies in seeds resulted in production levels between 1% and 8% of the total soluble protein. These in planta produced antibodies do not only bind to their purified antigens but also to Campylobacter bacterial cells. In addition, the anti-flagellin chimeric antibodies are reducing the motility of Campylobacter bacteria. These antibody-containing Arabidopsis seeds can be tested for oral passive immunisation of chickens and, if effective, the chimeric antibodies can be produced in crop seeds.

High accumulation in tobacco seeds of hemagglutinin antigen from avian (H5N1) influenza

Tobacco seeds can be used as a cost effective system for production of recombinant vaccines. Avian influenza is an important respiratory pathogen that causes a high degree of mortality and becomes a serious threat for the poultry industry. A safe vaccine against avian flu produced at low cost could help to prevent future outbreaks. We have genetically engineered tobacco plants to express extracellular domain of hemagglutinin protein from H5N1 avian influenza virus as an inexpensive alternative for production purposes. Two regulatory sequences of seed storage protein genes from Phaseolus vulgaris L. were used to direct the expression, yielding 3.0 mg of the viral antigen per g of seeds. The production and stability of seed-produced recombinant HA protein was characterized by different molecular techniques. The aqueous extract of tobacco seed proteins was used for subcutaneous immunization of chickens, which developed antibodies that inhibited the agglutination of erythrocytes after the second application of the antigen. The feasibility of using tobacco seeds as a vaccine carrier is discussed.

Enrichment of apoplastic fluid with therapeutic recombinant protein for efficient biofarming

OBJECTIVE

For efficient biofarming we attempted to enrich plant interstitial fluid (IF)/apoplastic fluid with targeted recombinant therapeutic protein. We employed a synthetic human Glucocerebrosidase (GCB), a model biopharmaceutical protein gene in this study.

RESULTS

Twenty one Nicotiana varieties, species and hybrids were initially screened for individual IF recovery and based on the findings, we selected Nicotiana tabacum NN (S-9-6), Nicotiana tabacum nn (S-9-7) and Nicotiana benthamiana (S-6-6) as model plants for raising transgenic expressing GCB via Agrobacterium mediated transformation under the control of M24 promoter; GCB specific activity in each transgenic lines were analyzed and we observed higher concentration of recombinant GCB in IF of these transgenic lines (S-9-6, S-9-7, and S-6-6) in comparison to their concentration in crude leaf extracts.

CONCLUSION

Recovery of valuable therapeutics in plant IF as shown in the present study holds great promise for promoting plant based biofarming. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:726-736, 2017.

Seed specific expression and analysis of recombinant human adenosine deaminase (hADA) in three host plant species

The plant seed is a leading platform amongst plant-based storage systems for the production of recombinant proteins. In this study, we compared the activity of human adenosine deaminase (hADA) expressed in transgenic seeds of three different plant species: pea (Pisum sativum L.), Nicotiana benthamiana L. and tarwi (Lupinus mutabilis Sweet). All three species were transformed with the same expression vector containing the hADA gene driven by the seed-specific promoter LegA2 with an apoplast targeting pinII signal peptide. During the study, several independent transgenic lines were generated and screened from each plant species and only lines with a single copy of the gene of interest were used for hADA expression analysis. A stable transgenic canola line expressing the ADA protein, under the control of 35S constitutive promoter was used as both as a positive control and for comparative study with the seed specific promoter. Significant differences were detected in the expression of hADA. The highest activity of the hADA enzyme (Units/g seed) was reported in tarwi (4.26 U/g) followed by pea (3.23 U/g) and Nicotiana benthamiana (1.69 U/g). The expression of mouse ADA in canola was very low in both seed and leaf tissue compared to other host plants, confirming higher activity of seed specific promoter. Altogether, these results suggest that tarwi could be an excellent candidate for the production of valuable recombinant proteins.

Plant Based Bioreactors of Recombinant Cytokines (Review)

Cytokines are a family of signaling polypeptides involved in intercellular interactions in the process of the immune response, as well as in the regulation of a number of normal physiological functions. Cytokines are used in medicine for the treatment of cancer, immune disorders, viral infections, and other socially significant diseases, but the extent of their use is limited by the high production cost of the active agent. The development of this area of pharmacology is associated with the success of genetic engineering, which allows the production of significant amounts of protein by transgenic organisms. The review discusses the latest advances in the production of various cytokines with the use of genetically modified plants.

Enhanced GAD65 production in plants using the MagnICON transient expression system: Optimization of upstream production and downstream processing

Plants have emerged as competitive production platforms for pharmaceutical proteins that are required in large quantities. One example is the 65-kDa isoform of human glutamic acid decarboxylase (GAD65), a major autoimmune diabetes autoantigen that has been developed as a vaccine candidate for the primary prevention of diabetes. The expression of GAD65 in plants has been optimized but large-scale purification is hampered by its tendency to associate with membranes. We investigated the potential for large-scale downstream processing by evaluating different combinations of plant-based expression systems and engineered forms of GAD65 in terms of yield, subcellular localization and solubility in detergent-free buffer. We found that a modified version of GAD65 lacking the first 87 amino acids accumulates to high levels in the cytosol and can be extracted in detergent-free buffer. The highest yields of this variant protein were achieved using the MagnICON transient expression system. This combination of truncated GAD65 and the MagnICON system dramatically boosts the production of the recombinant protein and helps to optimize downstream processing for the establishment of a sustainable plant-based production platform for an autoimmune diabetes vaccine candidate.

Androgenesis in Solanaceae

The Solanaceae is one of the most important families for global agriculture. Among the different solanaceous species, tobacco (Nicotiana tabacum), potato (Solanum tuberosum), tomato (Solanum lycopersicum), eggplant (Solanum melongena), and pepper (Capsicum annuum) are five crops of outstanding importance worldwide. In these crops, maximum yields are produced by hybrid plants created by crossing pure (homozygous) lines with the desired traits. Pure lines may be produced by conventional breeding methods, which is time consuming and costly. Alternatively, it is possible to accelerate the production of pure lines by creating doubled haploid (DH) plants derived from (haploid) male gametophytes or their precursors (androgenesis). In this way, the different steps for the production of pure lines can be reduced to only one generation, which implies important time and cost savings. This and other advantages make androgenic DHs the choice in a number of important crops where any of the different experimental in vitro techniques (anther culture or isolated microspore culture) is well set up. The Solanaceae family is an excellent example of heterogeneity in terms of response to these techniques, including highly responding species such as tobacco, considered a model system, and tomato, one of the most recalcitrant species, where no reliable and reproducible methods are yet available. Interestingly, the first evidence of androgenesis, particularly through in vitro anther culture, was demonstrated in a solanaceous species, Datura innoxia. In this chapter, we report the state of the art of the research about androgenic DHs in Solanaceae, paying special attention to datura, tobacco, potato, tomato, eggplant, and pepper.

Tobacco seeds as efficient production platform for a biologically active anti-HBsAg monoclonal antibody

The use of plants as heterologous hosts is one of the most promising technologies for manufacturing valuable recombinant proteins. Plant seeds, in particular, constitute ideal production platforms for long-term applications requiring a steady supply of starting material, as they combine the general advantages of plants as bioreactors with the possibility of biomass storage for long periods in a relatively small volume, thus allowing manufacturers to decouple upstream and downstream processing. In the present work we have used transgenic tobacco seeds to produce large amounts of a functionally active mouse monoclonal antibody against the Hepatitis B Virus surface antigen, fused to a KDEL endoplasmic reticulum retrieval motif, under control of regulatory sequences from common bean (Phaseolus vulgaris) seed storage proteins. The antibody accumulated to levels of 6.5 mg/g of seed in the T3 generation, and was purified by Protein A affinity chromatography combined with SEC-HPLC. N-glycan analysis indicated that, despite the KDEL signal, the seed-derived plantibody bore both high-mannose and complex-type sugars that indicate partial passage through the Golgi compartment, although its performance in the immunoaffinity purification of HBsAg was unaffected. An analysis discussing the industrial feasibility of replacing the currently used tobacco leaf-derived plantibody with this seed-derived variant is also presented.

Comparison of VHH-Fc antibody production in Arabidopsis thaliana, Nicotiana benthamiana and Pichia pastoris

VHHs or nanobodies are widely acknowledged as interesting diagnostic and therapeutic tools. However, for some applications, multivalent antibody formats, such as the dimeric VHH-Fc format, are desired to increase the functional affinity. The scope of this study was to compare transient expression of diagnostic VHH-Fc antibodies in Nicotiana benthamiana leaves with their stable expression in Arabidopsis thaliana seeds and Pichia pastoris. To this end, VHH-Fc antibodies targeting green fluorescent protein or the A. thaliana seed storage proteins (albumin and globulin) were produced in the three platforms. Differences were mainly observed in the accumulation levels and glycosylation patterns. Interestingly, although in plants oligomannosidic N-glycans were expected for KDEL-tagged VHH-Fcs, several VHH-Fcs with an intact KDEL-tag carried complex-type N-glycans, suggesting a dysfunctional retention in the endoplasmic reticulum. All VHH-Fcs were equally functional across expression platforms and several outperformed their corresponding VHH in terms of sensitivity in ELISA.

Recent advances on host plants and expression cassettes' structure and function in plant molecular pharming

Plant molecular pharming is a promising system to produce important recombinant proteins such as therapeutic antibodies, pharmaceuticals, enzymes, growth factors, and vaccines. The system provides an interesting alternative method to the direct extraction of proteins from inappropriate source material while offering the possibility to overcome problems related to product safety and source availability. Multiple factors including plant hosts, genes of interest, expression vector cassettes, and extraction and purification techniques play important roles in the plant molecular pharming. Plant species, as a biosynthesis platform, are a crucial factor in achieving high yields of recombinant protein in plant. The choice of recombinant gene and its expression strategy is also of great importance in ensuring a high amount of the recombinant proteins. Many studies have been conducted to improve expression, accumulation, and purification of the recombinant protein from molecular pharming systems. Re-engineered vectors and expression cassettes are also pivotal tools in enhancing gene expression at the transcription and translation level, and increasing protein accumulation, stability, retention and targeting of specific organelles. In this review, we report recent advances and strategies of plant molecular pharming while focusing on the choice of plant hosts and the role of some molecular pharming elements and approaches: promoters, codon optimization, signal sequences, and peptides used for upstream design, purification and downstream processing.

Production of cecropin A antimicrobial peptide in rice seed endosperm

BACKGROUND

Cecropin A is a natural antimicrobial peptide that exhibits rapid, potent and long-lasting lytic activity against a broad spectrum of pathogens, thus having great biotechnological potential. Here, we report a system for producing bioactive cecropin A in rice seeds.

RESULTS

Transgenic rice plants expressing a codon-optimized synthetic cecropin A gene drived by an endosperm-specific promoter, either the glutelin B1 or glutelin B4 promoter, were generated. The signal peptide sequence from either the glutelin B1 or the glutelin B4 were N-terminally fused to the coding sequence of the cecropin A. We also studied whether the presence of the KDEL endoplasmic reticulum retention signal at the C-terminal has an effect on cecropin A subcellular localization and accumulation. The transgenic rice plants showed stable transgene integration and inheritance. We show that cecropin A accumulates in protein storage bodies in the rice endosperm, particularly in type II protein bodies, supporting that the glutelin N-terminal signal peptides play a crucial role in directing the cecropin A to this organelle, independently of being tagged with the KDEL endoplasmic reticulum retention signal. The production of cecropin A in transgenic rice seeds did not affect seed viability or seedling growth. Furthermore, transgenic cecropin A seeds exhibited resistance to infection by fungal and bacterial pathogens (Fusarium verticillioides and Dickeya dadantii, respectively) indicating that the in planta-produced cecropin A is biologically active.

CONCLUSIONS

Rice seeds can sustain bioactive cecropin A production and accumulation in protein bodies. The system might benefit the production of this antimicrobial agent for subsequent applications in crop protection and food preservation.

Comparative analysis of different biofactories for the production of a major diabetes autoantigen

The 65-kDa isoform of human glutamic acid decarboxylase (hGAD65) is a major diabetes autoantigen that can be used for the diagnosis and (more recently) the treatment of autoimmune diabetes. We previously reported that a catalytically-inactive version (hGAD65mut) accumulated to tenfold higher levels than its active counterpart in transgenic tobacco plants, providing a safe and less expensive source of the protein compared to mammalian production platforms. Here we show that hGAD65mut is also produced at higher levels than hGAD65 by transient expression in Nicotiana benthamiana (using either the pK7WG2 or MagnICON vectors), in insect cells using baculovirus vectors, and in bacterial cells using an inducible-expression system, although the latter system is unsuitable because hGAD65mut accumulates within inclusion bodies. The most productive of these platforms was the MagnICON system, which achieved yields of 78.8 μg/g fresh leaf weight (FLW) but this was substantially less than the best-performing elite transgenic tobacco plants, which reached 114.3 μg/g FLW after six generations of self-crossing. The transgenic system was found to be the most productive and cost-effective although the breeding process took 3 years to complete. The MagnICON system was less productive overall, but generated large amounts of protein in a few days. Both plant-based systems were therefore advantageous over the baculovirus-based production platform in our hands.

Comparative evaluation of recombinant protein production in different biofactories: the green perspective

In recent years, the production of recombinant pharmaceutical proteins in heterologous systems has increased significantly. Most applications involve complex proteins and glycoproteins that are difficult to produce, thus promoting the development and improvement of a wide range of production platforms. No individual system is optimal for the production of all recombinant proteins, so the diversity of platforms based on plants offers a significant advantage. Here, we discuss the production of four recombinant pharmaceutical proteins using different platforms, highlighting from these examples the unique advantages of plant-based systems over traditional fermenter-based expression platforms.

Boosting in planta production of antigens derived from the porcine reproductive and respiratory syndrome virus (PRRSV) and subsequent evaluation of their immunogenicity

Porcine reproductive and respiratory syndrome (PRRS) is a disease of swine, caused by an arterivirus, the PRRS virus (PRRSV). This virus infects pigs worldwide and causes huge economic losses. Due to genetic drift, current vaccines are losing their power. Adaptable vaccines could provide a solution to this problem. This study aims at producing in planta a set of antigens derived from the PRRSV glycoproteins (GPs) to be included in a subunit vaccine. We selected the GP3, GP4 and GP5 and optimized these for production in an Arabidopsis seed platform by removing transmembrane domains (Tm) and/or adding stabilizing protein domains, such as the green fluorescent protein (GFP) and immunoglobulin (IgG) ‘Fragment crystallizable’ (Fc) chains. Accumulation of the GPs with and without Tm was low, reaching no more than 0.10% of total soluble protein (TSP) in homozygous seed. However, addition of stabilizing domains boosted accumulation up to a maximum of 2.74% of TSP when GFP was used, and albeit less effectively, also the Fc chains of the porcine IgG3 and murine IgG2a increased antigen accumulation, to 0.96% and 1.81% of TSP respectively, while the murine IgG3 Fc chain did not. Antigens with Tm were less susceptible to these manipulations to increase yield. All antigens were produced in the endoplasmic reticulum and accordingly, they carried high-mannose N-glycans. The immunogenicity of several of those antigens was assessed and we show that vaccination with purified antigens did elicit the production of antibodies with virus neutralizing activity in mice but not in pigs.

Trafficking of endoplasmic reticulum-retained recombinant proteins is unpredictable in Arabidopsis thaliana

A wide variety of recombinant proteins has been produced in the dicot model plant, Arabidopsis thaliana. Many of these proteins are targeted for secretion by means of an N-terminal endoplasmic reticulum (ER) signal peptide. In addition, they can also be designed for ER retention by adding a C-terminal H/KDEL-tag. Despite extensive knowledge of the protein trafficking pathways, the final protein destination, especially of such H/KDEL-tagged recombinant proteins, is unpredictable. In this respect, glycoproteins are ideal study objects. Microscopy experiments reveal their deposition pattern and characterization of their N-glycans aids in elucidating the trafficking. Here, we combine microscopy and N-glycosylation data generated in Arabidopsis leaves and seeds, and highlight the lack of a decent understanding of heterologous protein trafficking.

The dynamic behavior of storage organelles in developing cereal seeds and its impact on the production of recombinant proteins

Cereal endosperm is a highly differentiated tissue containing specialized organelles for the accumulation of storage proteins, which are ultimately deposited either within protein bodies derived from the endoplasmic reticulum, or in protein storage vacuoles (PSVs). During seed maturation endosperm cells undergo a rapid sequence of developmental changes, including extensive reorganization and rearrangement of the endomembrane system and protein transport via several developmentally regulated trafficking routes. Storage organelles have been characterized in great detail by the histochemical analysis of fixed immature tissue samples. More recently, in vivo imaging and the use of tonoplast markers and fluorescent organelle tracers have provided further insight into the dynamic morphology of PSVs in different cell layers of the developing endosperm. This is relevant for biotechnological applications in the area of molecular farming because seed storage organelles in different cereal crops offer alternative subcellular destinations for the deposition of recombinant proteins that can reduce proteolytic degradation, allow control over glycan structures and increase the efficacy of oral delivery. We discuss how the specialized architecture and developmental changes of the endomembrane system in endosperm cells may influence the subcellular fate and post-translational modification of recombinant glycoproteins in different cereal species.

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.

Plant species and organ influence the structure and subcellular localization of recombinant glycoproteins

Many plant-based systems have been developed as bioreactors to produce recombinant proteins. The choice of system for large-scale production depends on its intrinsic expression efficiency and its propensity for scale-up, post-harvest storage and downstream processing. Factors that must be considered include the anticipated production scale, the value and intended use of the product, the geographical production area, the proximity of processing facilities, intellectual property, safety and economics. It is also necessary to consider whether different species and organs affect the subcellular trafficking, structure and qualitative properties of recombinant proteins. In this article we discuss the subcellular localization and N-glycosylation of two commercially-relevant recombinant glycoproteins (Aspergillus niger phytase and anti-HIV antibody 2G12) produced in different plant species and organs. We augment existing data with novel results based on the expression of the same recombinant proteins in Arabidopsis and tobacco seeds, focusing on similarities and subtle differences in N-glycosylation that often reflect the subcellular trafficking route and final destination, as well as differences generated by unique enzyme activities in different species and tissues. We discuss the potential consequences of such modifications on the stability and activity of the recombinant glycoproteins.

Orally fed seeds producing designer IgAs protect weaned piglets against enterotoxigenic Escherichia coli infection

Oral feed-based passive immunization can be a promising strategy to prolong maternal lactogenic immunity against postweaning infections. Enterotoxigenic Escherichia coli (ETEC)-caused postweaning diarrhea in piglets is one such infection that may be prevented by oral passive immunization and might avert recurrent economic losses to the pig farming industry. As a proof of principle, we designed anti-ETEC antibodies by fusing variable domains of llama heavy chain-only antibodies (VHHs) against ETEC to the Fc part of a porcine immunoglobulin (IgG or IgA) and expressed them in Arabidopsis thaliana seeds. In this way, four VHH-IgG and four VHH-IgA antibodies were produced to levels of about 3% and 0.2% of seed weight, respectively. Cotransformation of VHH-IgA with the porcine joining chain and secretory component led to the production of light-chain devoid, assembled multivalent dimeric, and secretory IgA-like antibodies. In vitro analysis of all of the antibody-producing seed extracts showed inhibition of bacterial binding to porcine gut villous enterocytes. However, in the piglet feed-challenge experiment, only the piglets receiving feed containing the VHH-IgA-based antibodies (dose 20 mg/d per pig) were protected. Piglets receiving the VHH-IgA-based antibodies in the feed showed a progressive decline in shedding of bacteria, significantly lower immune responses corroborating reduced exposure to the ETEC pathogen, and a significantly higher weight gain compared with the piglets receiving VHH-IgG producing (dose 80 mg/d per pig) or wild-type seeds. These results stress the importance of the antibody format in oral passive immunization and encourage future expression of these antibodies in crop seeds.

High-level production of human interleukin-10 fusions in tobacco cell suspension cultures

The production of pharmaceutical proteins in plants has made much progress in recent years with the development of transient expression systems, transplastomic technology and humanizing glycosylation patterns in plants. However, the first therapeutic proteins approved for administration to humans and animals were made in plant cell suspensions for reasons of containment, rapid scale-up and lack of toxic contaminants. In this study, we have investigated the production of human interleukin-10 (IL-10) in tobacco BY-2 cell suspension and evaluated the effect of an elastin-like polypeptide tag (ELP) and a green fluorescent protein (GFP) tag on IL-10 accumulation. We report the highest accumulation levels of hIL-10 obtained with any stable plant expression system using the ELP fusion strategy. Although IL-10-ELP has cytokine activity, its activity is reduced compared to unfused IL-10, likely caused by interference of ELP with folding of IL-10. Green fluorescent protein has no effect on IL-10 accumulation, but examining the trafficking of IL-10-GFP over the cell culture cycle revealed fluorescence in the vacuole during the stationary phase of the culture growth cycle. Analysis of isolated vacuoles indicated that GFP alone is found in vacuoles, while the full-size fusion remains in the whole-cell extract. This indicates that GFP is cleaved off prior to its trafficking to the vacuole. On the other hand, IL-10-GFP-ELP remains mostly in the ER and accumulates to high levels. Protein bodies were observed at the end of the culture cycle and are thought to arise as a consequence of high levels of accumulation in the ER.

Transgenic rice seeds accumulating recombinant hypoallergenic birch pollen allergen Bet v 1 generate giant protein bodies

A versatile hypoallergenic allergen derivative against multiple allergens is an ideal tolerogen for allergen-specific immunotherapy. Such a tolerogen should exhibit high efficacy, without side effects, when administered at high doses and should be applicable to several allergens. Tree pollen chimera 7 (TPC7), a hypoallergenic Bet v 1 tolerogen against birch pollen allergy, was previously selected by DNA shuffling of 14 types of Fagales tree pollen allergens. In this study, transgenic rice seed accumulating TPC7 was generated as an oral vaccine against birch pollen allergy by expressing this protein as a secretory protein using the N-terminal signal peptide and the C-terminal KDEL tag under the control of an endosperm-specific glutelin promoter. The highest level of TPC7 accumulation was approximately 207 µg grain(-1). Recombinant TPC7 is a glycoprotein with high mannose-type N-glycan, but without β1,2-xylose or α1,3-fucose, suggesting that TPC7 is retained in the endoplasmic reticulum (ER). TPC7 is deposited as a novel, giant spherical ER-derived protein body, >20 µm in diameter, which is referred to as the TPC7 body. Removal of the KDEL retention signal or mutation of a cysteine residue resulted in an alteration of TPC7 body morphology, and deletion of the signal peptide prevented the accumulation of TPC7 in rice seeds. Therefore, the novel TPC7 bodies may have formed aggregates within the ER lumen, primarily due to the intrinsic physicochemical properties of the protein.

Recombinant antibody production in Arabidopsis seeds triggers an unfolded protein response

Among the many plant-based production systems that are being tested for molecular farming, seeds are very attractive, as they provide a stable environment in which the accumulating recombinant proteins can be stored. However, it is not known exactly how high production levels of recombinant antibodies influence the endogenous transcriptome and proteome of the developing seed. To address this question, we studied the transcriptomic status in developing Arabidopsis (Arabidopsis thaliana) seeds 13 d post anthesis of three transgenic lines, producing varying levels of recombinant VHH or single-chain Fv antibody fragments fused to the human immunoglobulin G1-derived Fc fragment under the control of the β-PHASEOLIN seed-specific promoter. Using genome-wide Tiling arrays, we demonstrated that only a small proportion of the transcriptome was significantly changed in each of the lines compared with the wild type. Strikingly, in all three lines, we found a large overlap of up-regulated genes corresponding to protein folding, glycosylation/modification, translocation, vesicle transport, and protein degradation, suggestive of a state of cellular stress called the unfolded protein response. Moreover, the gene up-regulation amplitude was similar in all three lines. We hypothesize that the production of recombinant antibodies in the endoplasmic reticulum triggers endoplasmic reticulum stress, causing a disturbance of the normal cellular homeostasis.

A noninvasive platform for imaging and quantifying oil storage in submillimeter tobacco seed

While often thought of as a smoking drug, tobacco (Nicotiana spp.) is now considered as a plant of choice for molecular farming and biofuel production. Here, we describe a noninvasive means of deriving both the distribution of lipid and the microtopology of the submillimeter tobacco seed, founded on nuclear magnetic resonance (NMR) technology. Our platform enables counting of seeds inside the intact tobacco capsule to measure seed sizes, to model the seed interior in three dimensions, to quantify the lipid content, and to visualize lipid gradients. Hundreds of seeds can be simultaneously imaged at an isotropic resolution of 25 µm, sufficient to assess each individual seed. The relative contributions of the embryo and the endosperm to both seed size and total lipid content could be assessed. The extension of the platform to a range of wild and cultivated Nicotiana species demonstrated certain evolutionary trends in both seed topology and pattern of lipid storage. The NMR analysis of transgenic tobacco plants with seed-specific ectopic expression of the plastidial phosphoenolpyruvate/phosphate translocator, displayed a trade off between seed size and oil concentration. The NMR-based assay of seed lipid content and topology has a number of potential applications, in particular providing a means to test and optimize transgenic strategies aimed at the manipulation of seed size, seed number, and lipid content in tobacco and other species with submillimeter seeds.

Expression and subcellular targeting of human complement factor C5a in Nicotiana species

We evaluated transgenic tobacco plants as an alternative to Escherichia coli for the production of recombinant human complement factor 5a (C5a). C5a has not been expressed in plants before and is highly unstable in vivo in its native form, so it was necessary to establish the most suitable subcellular targeting strategy. We used the strong and constitutive CaMV 35S promoter to drive transgene expression and compared three different subcellular compartments. The yields of C5a in the T(0) transgenic plants were low in terms of the proportion of total soluble protein (TSP) when targeted to the apoplast (0.0002% TSP) or endoplasmic reticulum (0.0003% TSP) but was one order of magnitude higher when targeted to the vacuole (0.001% TSP). The yields could be increased by conventional breeding (up to 0.014% TSP in the T₂ generation). C5a accumulated to the same level in seeds and leaves when targeted to the apoplast but was up to 1.7-fold more abundant in the seeds when targeted to the ER or vacuole, although this difference was less striking in the better-performing lines. When yields were calculated as an amount per gram fresh weight of transgenic plant tissue, the vacuole targeting strategy was clearly more efficient in seeds, reaching 35.8 µg C5a per gram of fresh seed weight compared to 10.62 µg C5a per gram fresh weight of leaves. Transient expression of C5aER and C5aVac in N. benthamiana, using MagnICON vectors, reached up to 0.2% and 0.7% of TSP, respectively, but was accompanied by cytotoxic effects and induced leaf senescence. Western blot of the plant extracts revealed a band matching the corresponding glycosylated native protein and the bioassay demonstrated that recombinant C5a was biologically active.

Genetic modification; the development of transgenic ornamental plant varieties

Plant transformation technology (hereafter abbreviated to GM, or genetic modification) has been used to develop many varieties of crop plants, but only a few varieties of ornamental plants. This disparity in the rate and extent of commercialisation, which has been noted for more than a decade, is not because there are no useful traits that can be engineered into ornamentals, is not due to market potential and is not due to a lack of research and development activity. The GM ornamental varieties which have been released commercially have been accepted in the marketplace. In this article, progress in the development of transgenic ornamentals is reviewed and traits useful to both consumers and producers are identified. In considering possible factors limiting the release of genetically modified ornamental products it is concluded that the most significant barrier to market is the difficulty of managing, and the high cost of obtaining, regulatory approval.

Recombinant cytokines from plants

Plant-based platforms have been successfully applied for the last two decades for the efficient production of pharmaceutical proteins. The number of commercialized products biomanufactured in plants is, however, rather discouraging. Cytokines are small glycosylated polypeptides used in the treatment of cancer, immune disorders and various other related diseases. Because the clinical use of cytokines is limited by high production costs they are good candidates for plant-made pharmaceuticals. Several research groups explored the possibilities of cost-effective production of animal cytokines in plant systems. This review summarizes recent advances in this field.