Plant-made pharmaceuticals: Leading products and production platforms

Genome editing for plant synthetic metabolic engineering and developmental regulation

Plant metabolism and development are a reflection of the orderly expression of genetic information intertwined with the environment interactions. Genome editing is the cornerstone for scientists to modify endogenous genes or introduce exogenous functional genes and metabolic pathways, holding immense potential applications in molecular breeding and biosynthesis. Over the course of nearly a decade of development, genome editing has advanced significantly beyond the simple cutting of double-stranded DNA, now enabling precise base and fragment replacements, regulation of gene expression and translation, as well as epigenetic modifications. However, the utilization of genome editing in plant synthetic metabolic engineering and developmental regulation remains exploratory. Here, we provide an introduction and a comprehensive overview of the editing attributes associated with various CRISPR/Cas tools, along with diverse strategies for the meticulous control of plant metabolic pathways and developments. Furthermore, we discuss the limitations of current approaches and future prospects for genome editing-driven plant breeding.

Plant-made pharmaceuticals: exploring studies for the production of recombinant protein in plants and assessing challenges ahead

The production of pharmaceutical compounds in plants is attracting increasing attention, as plant-based systems can be less expensive, safer, and more scalable than mammalian, yeast, bacterial, and insect cell expression systems. Here, we review the history and current status of plant-made pharmaceuticals. Producing pharmaceuticals in plants requires pairing the appropriate plant species with suitable transformation technology. Pharmaceuticals have been produced in tobacco, cereals, legumes, fruits, and vegetables via nuclear transformation, chloroplast transformation, transient expression, and transformation of suspension cell cultures. Despite this wide range of species and methods used, most such efforts have involved the nuclear transformation of tobacco. Tobacco readily generates large amounts of biomass, easily accepts foreign genes, and is amenable to stable gene expression via nuclear transformation. Although vaccines, antibodies, and therapeutic proteins have been produced in plants, such pharmaceuticals are not readily utilized by humans due to differences in glycosylation, and few such compounds have been approved due to a lack of clinical data. In addition, achieving an adequate immune response using plant-made pharmaceuticals can be difficult due to low rates of production compared to other expression systems. Various technologies have recently been developed to help overcome these limitations; however, plant systems are expected to increasingly become widely used expression systems for recombinant protein production.

The production of the first functional antibody mimetic in higher plants: the chloroplast makes the DARPin G3 for HER2 imaging in oncology

Background

Designed mimetic molecules are attractive tools in biopharmaceuticals and synthetic biology. They require mass and functional production for the assessment of upcoming challenges in the near future. The DARPin family is considered a mimetic pharmaceutical peptide group with high affinity binding to specific targets. DARPin G3 is designed to bind to the HER2 (human epidermal growth factor receptor 2) tyrosine kinase receptor. Overexpression of HER2 is common in some cancers, including breast cancer, and can be used as a prognostic and predictive tool for cancer. The chloroplasts are cost-effective alternatives, equal to, and sometimes better than, bacterial, yeast, or mammalian expression systems. This research examined the possibility of the production of the first antibody mimetic, DARPin G3, in tobacco chloroplasts for HER2 imaging in oncology.

Results

The chloroplast specific DARPin G3 expression cassette was constructed and transformed into N. tabacum chloroplasts. PCR and Southern blot analysis confirmed integration of transgenes as well as chloroplastic and cellular homoplasmy. The Western blot analysis and ELISA confirmed the production of DARPin G3 at the commercial scale and high dose with the rate of 20.2% in leaf TSP and 33.7% in chloroplast TSP. The functional analysis by ELISA confirmed the binding of IMAC purified chloroplast-made DARPin G3 to the extracellular domain of the HER2 receptor with highly effective picomolar affinities. The carcinoma cellular studies by flow cytometry and immunofluorescence microscopy confirmed the correct functioning by the specific binding of the chloroplast-made DARPin G3 to the HER2 receptor on the surface of HER2-positive cancer cell lines.

Conclusion

The efficient functional bioactive production of DARPin G3 in chloroplasts led us to introduce plant chloroplasts as the site of efficient production of the first antibody mimetic molecules. This report, as the first case of the cost-effective production of mimetic molecules, enables researchers in pharmaceuticals, synthetic biology, and bio-molecular engineering to develop tool boxes by producing new molecular substitutes for diverse purposes.

Advances in engineering the production of the natural red pigment lycopene: A systematic review from a biotechnology perspective

BACKGROUND

Lycopene is a natural red compound with potent antioxidant activity that can be utilized both as pigment and as a raw material in functional food, and so possesses good commercial prospects. The biosynthetic pathway has already been documented, which provides the foundation for lycopene production using biotechnology.

AIM OF REVIEW

Although lycopene production has begun to take shape, there is still an urgent need to alleviate the yield of lycopene. Progress in this area can provide useful reference for metabolic engineering of lycopene production utilizing multiple approaches. Key scientific concepts of review Using conventional microbial fermentation approaches, biotechnologists have enhanced the yield of lycopene by selecting suitable host strains, utilizing various additives, and optimizing culture conditions. With the development of modern biotechnology, genetic engineering, protein engineering, and metabolic engineering have been applied for lycopene production. Extraction from natural plants is the main way for lycopene production at present. Based on the molecular mechanism of lycopene accumulation, the production of lycopene by plant bioreactor through genetic engineering has a good prospect. Here we summarized common strategies for optimizing lycopene production engineering from a biotechnology perspective, which are mainly carried out by microbial cultivation. We reviewed the challenges and limitations of this approach, summarized the critical aspects, and provided suggestions with the aim of potential future breakthroughs for lycopene production in plants.

Successful Production and Ligninolytic Activity of a Bacterial Laccase, Lac51, Made in Nicotiana benthamiana via Transient Expression

Giant panda could have bamboo as their exclusive diet for about 2 million years because of the contribution of numerous enzymes produced by their gut bacteria, for instance laccases. Laccases are blue multi-copper oxidases that catalyze the oxidation of a broad spectrum of phenolic and aromatic compounds with water as the only byproduct. As a "green enzyme," laccases have potential in industrial applications, for example, when dealing with degradation of recalcitrant biopolymers, such as lignin. In the current study, a bacterial laccase, Lac51, originating from Pseudomonas putida and identified in the gut microbiome of the giant panda's gut was transiently expressed in the non-food plant Nicotiana benthamiana and characterized. Our results show that recombinant Lac51 exhibits bacterial laccase properties, with optimal pH and temperature at 7-8 and 40°C, respectively, when using syringaldazine as substrate. Moreover, we demonstrate the functional capability of the plant expressed Lac51 to oxidize lignin using selected lignin monomers that serve as substrates of Lac51. In summary, our study demonstrates the potential of green and non-food plants as a viable enzyme production platform for bacterial laccases. This result enriches our understanding of plant-made enzymes, as, to our knowledge, Lac51 is the first functional recombinant laccase produced in plants.

Molecular farming using transgenic rice endosperm

Plant expression platforms are low-cost, scalable, safe, and environmentally friendly systems for the production of recombinant proteins and bioactive metabolites. Rice (Oryza sativa L.) endosperm is an ideal bioreactor for the production and storage of high-value active substances, including pharmaceutical proteins, oral vaccines, vitamins, and nutraceuticals such as flavonoids and carotenoids. Here, we explore the use of molecular farming from producing medicines to developing functional food crops (biofortification). We review recent progress in producing pharmaceutical proteins and bioactive substances in rice endosperm and compare this platform with other plant expression systems. We describe how rice endosperm could be modified to design metabolic pathways and express and store stable products and discuss the factors restricting the commercialization of transgenic rice products and future prospects.

Production of recombinant β-glucocerebrosidase in wild-type and glycoengineered transgenic Nicotiana benthamiana root cultures with different N-glycan profiles

Gaucher disease is an inherited lysosomal storage disorder caused by an insufficiency of active β-glucocerebrosidase (GCase). Exogenous recombinant GCase via enzyme replacement therapy is considered the most practical treatment for Gaucher disease. Mannose receptors mediate the efficient uptake of exogenous GCase into macrophages. Thus, terminal mannose residues on N-glycans are essential for the delivery of exogenous GCase. In this study, recombinant GCase was produced in root cultures of wild-type (WT) and glycoengineered transgenic Nicotiana benthamiana with downregulated N-acetylglucosaminyltransferase I expression. Root cultures of WT and glycoengineered transgenic N. benthamiana plants were successfully generated by the induction of plant hormones. Recombinant GCases produced in both root cultures possessed GCase enzyme activity. Purified GCases derived from both root cultures revealed different N-glycan profiles. The WT-derived GCase possessed the predominant plant-type N-glycans, which contain plant-specific sugars-linkages, specifically β1,2-xylose and α1,3-fucose residues. Notably, the mannosidic-type N-glycans with terminal mannose residues were abundant in the purified GCase derived from glycoengineered N. benthamiana root culture. This research provides a promising plant-based system for the production of recombinant GCase with terminal mannose residues on N-glycans.

Compendium on Food Crop Plants as a Platform for Pharmaceutical Protein Production

Tremendous advances in crop biotechnology related to the availability of molecular tools and methods developed for transformation and regeneration of specific plant species have been observed. As a consequence, the interest in plant molecular farming aimed at producing the desired therapeutic proteins has significantly increased. Since the middle of the 1980s, recombinant pharmaceuticals have transformed the treatment of many serious diseases and nowadays are used in all branches of medicine. The available systems of the synthesis include wild-type or modified mammalian cells, plants or plant cell cultures, insects, yeast, fungi, or bacteria. Undeniable benefits such as well-characterised breeding conditions, safety, and relatively low costs of production make plants an attractive yet competitive platform for biopharmaceutical production. Some of the vegetable plants that have edible tubers, fruits, leaves, or seeds may be desirable as inexpensive bioreactors because these organs can provide edible vaccines and thus omit the purification step of the final product. Some crucial facts in the development of plant-made pharmaceuticals are presented here in brief. Although crop systems do not require more strictly dedicated optimization of methodologies at any stages of the of biopharmaceutical production process, here we recall the complete framework of such a project, along with theoretical background. Thus, a brief review of the advantages and disadvantages of different systems, the principles for the selection of cis elements for the expression cassettes, and available methods of plant transformation, through to the protein recovery and purification stage, are all presented here. We also outline the achievements in the production of biopharmaceuticals in economically important crop plants and provide examples of their clinical trials and commercialization.

Production of Designer VHH-Based Antibodies in Plants

Simplified monoclonal antibodies can be produced by fusing a VHH or nanobody, derived from camelid heavy-chain-only antibodies to the Fc domain of either IgG (VHH-IgG), IgA (VHH-IgA), or IgY (VHH-IgY). These recombinant antibodies are encoded by a single gene and their production can be easily scaled up in plants. This chapter contains methods for Gateway cloning of VHH-Fc fusions into the binary T-DNA vectors pEAQ-HT-DEST1 and pPhasGW, electroporation of Agrobacterium with the resulting constructs, transient antibody expression in Nicotiana benthamiana leaves, and stable antibody expression in Arabidopsis thaliana seeds. The properties of chimeric VHH-based antibodies produced in plants enable novel passive immunization treatments, such as in-feed oral delivery or intravenous injection.

Efficacy of the commercial plant products acting against influenza-a review

Background

Influenza infection always poses a threat to human and animal health. Vaccines and antiviral drugs are recommended to deal with the situation. The drawback of these remedial agents made the scientist change their focus on an alternative therapy. The anti-influenza effects of plants have been extensively studied, and many pharmaceutical companies have prepared their products on this basis.

Main body

The present review documents the successfully launched anti-influenza commercial products. In specific, it exposes the scientifically validated and evidence-based supporting inhibitory action of influenza and its strains.

Conclusion

This review highlighted the efficacy of the commercial products which effectively combat influenza. It provides a complementary strategy to deal with the worst-case scenario of flu. Meanwhile, to face the emerging strains, brand new products are in great necessity besides prevailing and available drugs.

Plant-Expressed Receptor Binding Domain of the SARS-CoV-2 Spike Protein Elicits Humoral Immunity in Mice

The current 15-month coronavirus disease-19 (COVID-19) pandemic caused by SARS-CoV-2 has accounted for 3.77 million deaths and enormous worldwide social and economic losses. A high volume of vaccine production is urgently required to eliminate COVID-19. Inexpensive and robust production platforms will improve the distribution of vaccines to resource-limited countries. Plant species offer such platforms, particularly through the production of recombinant proteins to serve as immunogens. To achieve this goal, here we expressed the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) protein in the glycoengineered-tobacco plant Nicotiana benthamiana to provide a candidate subunit vaccine. This recombinant RBD elicited humoral immunity in mice via induction of highly neutralizing antibodies. These findings provide a strong foundation to further advance the development of plant-expressed RBD antigens for use as an effective, safe, and inexpensive SARS-CoV-2 vaccine. Moreover, our study further highlights the utility of plant species for vaccine development.

Plant-Produced Receptor-Binding Domain of SARS-CoV-2 Elicits Potent Neutralizing Responses in Mice and Non-human Primates

The emergence of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected global public health and economy. Despite the substantial efforts, only few vaccines are currently approved and some are in the different stages of clinical trials. As the disease rapidly spreads, an affordable and effective vaccine is urgently needed. In this study, we investigated the immunogenicity of plant-produced receptor-binding domain (RBD) of SARS-CoV-2 in order to use as a subunit vaccine. In this regard, RBD of SARS-CoV-2 was fused with Fc fragment of human IgG1 and transiently expressed in Nicotiana benthamiana by agroinfiltration. The plant-produced RBD-Fc fusion protein was purified from the crude extract by using protein A affinity column chromatography. Two intramuscular administration of plant-produced RBD-Fc protein formulated with alum as an adjuvant have elicited high neutralization titers in immunized mice and cynomolgus monkeys. Further it has induced a mixed Th1/Th2 immune responses and vaccine-specific T-lymphocyte responses which was confirmed by interferon-gamma (IFN-γ) enzyme-linked immunospot assay. Altogether, our results demonstrated that the plant-produced SARS-CoV-2 RBD has the potential to be used as an effective vaccine candidate against SARS-CoV-2. To our knowledge, this is the first report demonstrating the immunogenicity of plant-produced SARS-CoV-2 RBD protein in mice and non-human primates.

Bioproduction process of natural products and biopharmaceuticals: Biotechnological aspects

Decades of research have been put in place for developing sustainable routes of bioproduction of high commercial value natural products (NPs) on the global market. In the last few years alone, we have witnessed significant advances in the biotechnological production of NPs. The development of new methodologies has resulted in a better understanding of the metabolic flux within the organisms, which have driven manipulations to improve production of the target product. This was further realised due to the recent advances in the omics technologies such as genomics, transcriptomics, proteomics, metabolomics and secretomics, as well as systems and synthetic biology. Additionally, the combined application of novel engineering strategies has made possible avenues for enhancing the yield of these products in an efficient and economical way. Invention of high-throughput technologies such as next generation sequencing (NGS) and toolkits for genome editing Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated 9 (CRISPR/Cas9) have been the game changers and provided unprecedented opportunities to generate rationally designed synthetic circuits which can produce complex molecules. This review covers recent advances in the engineering of various hosts for the production of bioactive NPs and biopharmaceuticals. It also highlights general approaches and strategies to improve their biosynthesis with higher yields in a perspective of plants and microbes (bacteria, yeast and filamentous fungi). Although there are numerous reviews covering this topic on a selected species at a time, our approach herein is to give a comprehensive understanding about state-of-art technologies in different platforms of organisms.

Plant-made immunotoxin building blocks: A roadmap for producing therapeutic antibody-toxin fusions

Molecular farming in plants is an emerging platform for the production of pharmaceutical proteins, and host species such as tobacco are now becoming competitive with commercially established production hosts based on bacteria and mammalian cell lines. The range of recombinant therapeutic proteins produced in plants includes replacement enzymes, vaccines and monoclonal antibodies (mAbs). But plants can also be used to manufacture toxins, such as the mistletoe lectin viscumin, providing an opportunity to express active antibody-toxin fusion proteins, so-called recombinant immunotoxins (RITs). Mammalian production systems are currently used to produce antibody-drug conjugates (ADCs), which require the separate expression and purification of each component followed by a complex and hazardous coupling procedure. In contrast, RITs made in plants are expressed in a single step and could therefore reduce production and purification costs. The costs can be reduced further if subcellular compartments that accumulate large quantities of the stable protein are identified and optimal plant growth conditions are selected. In this review, we first provide an overview of the current state of RIT production in plants before discussing the three key components of RITs in detail. The specificity-defining domain (often an antibody) binds cancer cells, including solid tumors and hematological malignancies. The toxin provides the means to kill target cells. Toxins from different species with different modes of action can be used for this purpose. Finally, the linker spaces the two other components to ensure they adopt a stable, functional conformation, and may also promote toxin release inside the cell. Given the diversity of these components, we extract broad principles that can be used as recommendations for the development of effective RITs. Future research should focus on such proteins to exploit the advantages of plants as efficient production platforms for targeted anti-cancer therapeutics.

Transient Production of Human β-Glucocerebrosidase With Mannosidic-Type N-Glycan Structure in Glycoengineered Nicotiana benthamiana Plants

Gaucher disease is an inherited lysosomal storage disorder caused by a deficiency of functional enzyme β-glucocerebrosidase (GCase). Recombinant GCase has been used in enzyme replacement therapy to treat Gaucher disease. Importantly, the terminal mannose N-glycan structure is essential for the uptake of recombinant GCase into macrophages via the mannose receptor. In this research, recombinant GCase was produced using Agrobacterium-mediated transient expression in both wild-type (WT) and N-acetylglucosaminyltransferase I (GnTI) downregulated Nicotiana benthamiana (ΔgntI) plants, the latter of which accumulates mannosidic-type N-glycan structures. The successfully produced functional GCase exhibited GCase enzyme activity. The enzyme activity was the same as that of the conventional mammalian-derived GCase. Notably, N-glycan analysis revealed that a mannosidic-type N-glycan structure lacking plant-specific N-glycans (β1,2-xylose and α1,3-fucose residues) was predominant in all glycosylation sites of purified GCase produced from ΔgntI plants. Our research provides a promising alternative plant line as a host for the production of recombinant GCase with a mannosidic-type N-glycan structure. This glycoengineered plant might be applicable to the production of other pharmaceutical proteins, especially mannose receptor targeted protein, for therapeutic uses.

Plant-Based Drugs and Vaccines for COVID-19

The coronavirus SARS-CoV-2 has turned our own health and the world economy upside down. While several vaccine candidates are currently under development, antivirals with the potential to limit virus transmission or block infection are also being explored. Plant production platforms are being used to generate vaccines and antiviral proteins inexpensively and at mass scale. The following review discusses the biology and origins of the current coronavirus pandemic, and describes some of the conventional, synthetic, and plant-based approaches to address the challenge that it presents to our way of life.

Optimization of culture parameters for α-glucosidase production from suspension culture of moss Hyophilla nymaniana (Fleish.) Menzel

BACKGROUND

Bryophytes, comprising of the second largest group in the plant kingdom, has attracted a great deal of attention in recent years due to its immense potential to produce biopharmaceuticals. But studies conducted to better understand their chemical composition are limited and scattered. In the present investigation, sequential optimization strategy, based on statistical experimental designs, was employed to enhance the production of α-glucosidase enzyme from moss Hyophilla nymaniana (Fleish.) Menzel by using Taguchi methodology. L16 orthogonal array and five physical parameters including sugar, temperature, pH, rpm, nitrogen source were considered as key parameters for enzyme production. The optimal level of each parameter for maximum glucosidase production by the moss was determined. Analysis of variance (ANOVA) was performed to evaluate statistically significant process factors.

RESULTS

Based on statistical analysis (ANOVA), the optimal combinations of the major constituents of media for maximal α-glucosidase production were evaluated as follows: dextrose 2% contributed maximum on α-glucosidase production followed by ammonium nitrate 1.5%, temperature 24 °C, pH 5.6, and RPM 120. Predicted results showed an enhanced glucosidase (53%) than the basal production medium.

CONCLUSION

The present study highlighted that Taguchi design of experiments approach is better than the conventional optimization technique to determine the optimum level of each of the significant parameters that brings maximum enzyme production.

Plant cell-secreted stem cell factor stimulates expansion and differentiation of hematopoietic stem cells

Ex vivo generation of red blood cells (RBCs) from hematopoietic stem cells (HSCs) used for blood transfusion represents one of the focuses in current regenerative medicine. However, massive production of HSCs-based RBCs requires a significant quantity of erythropoietic growth factors, making manufacturing at large scale cost prohibitive. Plant cell culture is proposed to be a promising bioproduction platform for functional human proteins in a safe and cost-efficient manner. This study exploited a proprietary technology, named HypGP engineering technology, for high-yield production of one of the key erythropoietic growth factors--stem cell factor (SCF)--in plant cell culture. Specifically, a designer hydroxyproline (Hyp)-O-glycosylated peptide (HypGP) comprised of 20 tandem repeats of the "Ser-Pro" motif, or (SP)_20, was engineered at either the N-terminus or C-terminus of SCF in tobacco BY-2 cells. The (SP)₂₀ tag dramatically increased the secreted yields of SCF up to 2.5 μg/ml. The (SP)₂₀-tagged SCF showed bioactivity in promoting the proliferation of the TF-1 cell line, although the SCF-(SP)₂₀ was 8.4-fold more potent than the (SP)₂₀-SCF. Both the (SP)₂₀-SCF and SCF-(SP)₂₀ exhibited desired function in stimulating the expansion and differentiation of human umbilical cord blood CD34⁺ cells towards RBCs.

Bulaon CJ, Phoolcharoen W. Plant Molecular Farming: A Viable Platform for Recombinant Biopharmaceutical Production

The demand for recombinant proteins in terms of quality, quantity, and diversity is increasing steadily, which is attracting global attention for the development of new recombinant protein production technologies and the engineering of conventional established expression systems based on bacteria or mammalian cell cultures. Since the advancements of plant genetic engineering in the 1980s, plants have been used for the production of economically valuable, biologically active non-native proteins or biopharmaceuticals, the concept termed as plant molecular farming (PMF). PMF is considered as a cost-effective technology that has grown and advanced tremendously over the past two decades. The development and improvement of the transient expression system has significantly reduced the protein production timeline and greatly improved the protein yield in plants. The major factors that drive the plant-based platform towards potential competitors for the conventional expression system are cost-effectiveness, scalability, flexibility, versatility, and robustness of the system. Many biopharmaceuticals including recombinant vaccine antigens, monoclonal antibodies, and other commercially viable proteins are produced in plants, some of which are in the pre-clinical and clinical pipeline. In this review, we consider the importance of a plant- based production system for recombinant protein production, and its potential to produce biopharmaceuticals is discussed.

Caffeic Acid Phenethyl Ester Assisted by Reversible Electroporation-In Vitro Study on Human Melanoma Cells

Melanoma is one of the most serious skin cancers. The incidence of this malignant skin lesion is continuing to increase worldwide. Melanoma is resistant to chemotherapeutic drugs and highly metastatic. Surgical resection can only be used to treat melanoma in the early stages, while chemotherapy is limited due to melanoma multi-drug resistance. The overexpression of glutathione S-transferase (GST) may have a critical role in this resistance. Caffeic acid phenethyl ester (CAPE) is a natural phenolic compound, which occurs in many plants. Previous studies demonstrated that CAPE suppresses the growth of melanoma cells and induces reactive oxygen species generation. It is also known that bioactivation of CAPE to its corresponding quinone metabolite by tyrosinase would lead to GST inhibition and selective melanoma cell death. We investigated the biochemical toxicity of CAPE in combination with microsecond electropermeabilization in two human melanoma cell lines. Our results indicate that electroporation of melanoma cells in the presence of CAPE induced high oxidative stress, which correlates with high cytotoxicity. Moreover, it can disrupt the metabolism of cancer cells by inducing apoptotic cell death. Electroporation of melanoma cells may be an efficient CAPE delivery system, enabling the application of this compound, while reducing its dose and exposure time.

Emergence of Novel Coronavirus 2019-nCoV: Need for Rapid Vaccine and Biologics Development

Novel Coronavirus (2019-nCoV) is an emerging pathogen that was first identified in Wuhan, China in late December 2019. This virus is responsible for the ongoing outbreak that causes severe respiratory illness and pneumonia-like infection in humans. Due to the increasing number of cases in China and outside China, the WHO declared coronavirus as a global health emergency. Nearly 35,000 cases were reported and at least 24 other countries or territories have reported coronavirus cases as early on as February. Inter-human transmission was reported in a few countries, including the United States. Neither an effective anti-viral nor a vaccine is currently available to treat this infection. As the virus is a newly emerging pathogen, many questions remain unanswered regarding the virus’s reservoirs, pathogenesis, transmissibility, and much more is unknown. The collaborative efforts of researchers are needed to fill the knowledge gaps about this new virus, to develop the proper diagnostic tools, and effective treatment to combat this infection. Recent advancements in plant biotechnology proved that plants have the ability to produce vaccines or biopharmaceuticals rapidly in a short time. In this review, the outbreak of 2019-nCoV in China, the need for rapid vaccine development, and the potential of a plant system for biopharmaceutical development are discussed.

Plant based production of myoglobin - a novel source of the muscle heme-protein

Myoglobin is a heme-protein in the muscle of vertebrates with important functions in the oxygenation of tissues and as a regulator in nitric oxide signaling. Myoglobin from many species is also an important nutritional source of bioavailable iron. In this study, we have successfully produced human myoglobin in the leaves of Nicotiana benthamiana by transient expression using a viral vector delivered by Agrobacterium tumefaciens. Analyses confirmed that heme was incorporated and the protein was functional, with observed properties consistent with those of native myoglobins. A relatively high degree of purity could be achieved with low cost methods. The results show the high potential of plants as a production platform for heme proteins, a group of proteins of interest for iron nutrition applications and possible future pharmaceutical development.

Seeds as Economical Production Platform for Recombinant Proteins

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The cost-effective production of high-quality and biologically active recombinant molecules especially proteins is extremely desirable. Seed-based recombinant protein production platforms are considered as superior choice owing to lack of human/animal pathogenic organisms, lack of cold chain requirements for transportation and long-term storage, easy scalability and development of edible biopharmaceuticals in plants with objective to be used in purified or partially processed form is desirable. This review article summarizes the exceptional features of seed-based biopharming and highlights the needs of exploiting it for commercial purposes. Plant seeds offer a perfect production platform for high-value molecules of industrial as well as therapeutic nature owing to lower water contents, high protein storage capacity, weak protease activity and long-term storage ability at ambient temperature. Exploiting extraordinarily high protein accumulation potential, vaccine antigens, antibodies and other therapeutic proteins can be stored without effecting their stability and functionality up to years in seeds. Moreover, ability of direct oral consumption and post-harvest stabilizing effect of seeds offer unique feature of oral delivery of pharmaceutical proteins and vaccine antigens for immunization and disease treatment through mucosal as well as oral route.

Expression, Purification and Initial Characterization of Functional α1-Microglobulin (A1M) in Nicotiana benthamiana

α₁-Microglobulin (A1M) is a small glycoprotein that belongs to the lipocalin protein family. A major biological role of A1M is to protect cells and tissues against oxidative damage by clearing free heme and reactive oxygen species. Because of this, the protein has attracted great interest as a potential pharmaceutical candidate for treatment of acute kidney injury and preeclampsia. The aim of this study was to explore the possibility of expressing human A1M in plants through transient gene expression, as an alternative or complement to other expression systems. E. coli, insect and mammalian cell culture have previously been used for recombinant A1M (rA1M) or A1M production, but these systems have various drawbacks, including additional complication and expense in refolding for E. coli, while insect produced rA1M is heavily modified with chromophores and mammalian cell culture has been used only in analytical scale. For that purpose, we have used a viral vector (pJL-TRBO) delivered by Agrobacterium for expression of three modified A1M gene variants in the leaves of N. benthamiana. The results showed that these modified rA1M protein variants, A1M-NB1, A1M-NB2 and A1M-NB3, targeted to the cytosol, ER and extracellular space, respectively, were successfully expressed in the leaves, which was confirmed by SDS-PAGE and Western blot analysis. The cytosol accumulated A1M-NB1 was selected for further analysis, as it appeared to have a higher yield than the other variants, and was purified with a yield of ca. 50 mg/kg leaf. The purified protein had the expected structural and functional properties, displaying heme-binding capacity and capacity of protecting red blood cells against stress-induced cell death. The protein also carried bound chromophores, a characteristic feature of A1M and an indicator of a capacity to bind small molecules. The study showed that expression of the functional protein in N. benthamiana may be an attractive alternative for production of rA1M for pharmaceutical purposes and a basis for future research on A1M structure and function.

Surfing the Blood Coagulation Cascade: Insight into the Vital Factor Xa

Factor Xa (FXa) plays a key role in haemostasis, it is a central part of the blood coagulation cascade which catalyzes the production of thrombin and leads to clot formation and wound closure. Therefore, FXa is an attractive target for the development of new anticoagulant agents. In this review, we will first describe the molecular features of this fundamental protein in order to understand its mechanism of action, an essential background for the design of novel inhibitors by means of synthetic organic chemistry or using peptides obtained from recombinant methodologies. Then, we will review the current state of the synthesis of novel direct FXa inhibitors along with their mechanisms of action. Finally, approved reversal agents that aid in maintaining blood haemostasis by using these commercial drugs will also be 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.

Assessing Carnivorous Plants for the Production of Recombinant Proteins

The recovery of recombinant proteins from plant tissues is an expensive and time-consuming process involving plant harvesting, tissue extraction, and subsequent protein purification. The downstream process costs can represent up to 80% of the total cost of production. Secretion-based systems of carnivorous plants might help circumvent this problem. Drosera and Nepenthes can produce and excrete out of their tissues a digestive fluid containing up to 200 mg. L-1 of natural proteins. Based on the properties of these natural bioreactors, we have evaluated the possibility to use carnivorous plants for the production of recombinant proteins. In this context, we have set up original protocols of stable and transient genetic transformation for both Drosera and Nepenthes sp. The two major drawbacks concerning the proteases naturally present in the secretions and a polysaccharidic network composing the Drosera glue were overcome by modulating the pH of the plant secretions. At alkaline pH, digestive enzymes are inactive and the interactions between the polysaccharidic network and proteins in the case of Drosera are subdued allowing the release of the recombinant proteins. For D. capensis, a concentration of 25 μg of GFP/ml of secretion (2% of the total soluble proteins from the glue) was obtained for stable transformants. For N. alata, a concentration of 0.5 ng of GFP/ml secretions (0.5% of total soluble proteins from secretions) was reached, corresponding to 12 ng in one pitcher after 14 days for transiently transformed plants. This plant-based expression system shows the potentiality of biomimetic approaches leading to an original production of recombinant proteins, although the yields obtained here were low and did not allow to qualify these plants for an industrial platform project.

Plant-Made Antibodies: Properties and Therapeutic Applications

BACKGROUND

A cost-effective plant platform for therapeutic monoclonal antibody production is both flexible and scalable. Plant cells have mechanisms for protein synthesis and posttranslational modification, including glycosylation, similar to those in animal cells. However, plants produce less complex and diverse Asn-attached glycans compared to animal cells and contain plant-specific residues. Nevertheless, plant-made antibodies (PMAbs) could be advantageous compared to those produced in animal cells due to the absence of a risk of contamination from nucleic acids or proteins of animal origin.

OBJECTIVE

In this review, the various platforms of PMAbs production are described, and the widely used transient expression system based on Agrobacterium-mediated delivery of genetic material into plant cells is discussed in detail.

RESULTS

We examined the features of and approaches to humanizing the Asn-linked glycan of PMAbs. The prospects for PMAbs in the prevention and treatment of human infectious diseases have been illustrated by promising results with PMAbs against human immunodeficiency virus, rotavirus infection, human respiratory syncytial virus, rabies, anthrax and Ebola virus. The pre-clinical and clinical trials of PMAbs against different types of cancer, including lymphoma and breast cancer, are addressed.

CONCLUSION

PMAb biosafety assessments in patients suggest that it has no side effects, although this does not completely remove concerns about the potential immunogenicity of some plant glycans in humans. Several PMAbs at various developmental stages have been proposed. Promise for the clinical use of PMAbs is aimed at the treatment of viral and bacterial infections as well as in anti-cancer treatment.

Production of functional human interleukin 37 using plants

KEY MESSAGE

We demonstrate for the first time that a fully bioactive human IL-37, a newly discovered cytokine acting as a fundamental inhibitor of innate immunity, can be recombinantly produced in plant cells. Interleukin 37 (IL-37), a newly discovered member of the interleukin (IL)-1 family of cytokines, plays a pivotal role in limiting innate inflammation and suppressing acquired immune responses, thus holding high potential for treating a wide array of human inflammatory and autoimmune disorders. In this study, we have developed transgenic plants as a novel expression platform for production of human IL-37 (IL-37). Plant transformation vectors synthesizing various forms of the b isoform of IL-37, including an unprocessed full-length precursor form (proIL-37b), a mature form (matIL-37b) and an IL-37 fusion protein in which IL-37b was fused to soybean agglutinin (SBA-IL-37b), have been constructed and introduced into tobacco plants. The expression of all forms of IL-37b was driven by a strong constitutive 35S promoter. Transgenic tobacco plants were generated with each of these constructs. Depending on the form of IL-37b being produced, the expression level of proIL-37b reached approximately 1% of TSP, while matIL-37b expression was substantially lower (0.01% TSP). Fusion to SBA substantially increased the expression of matIL-37b, with the expression level of fusion protein accounting for 1% of TSP. Functional analysis using a cell-based in vitro assay showed that plant-made matIL-37b and proIL-37b are both biologically active, but plant-made matIL-37b exhibited significantly greater biological activity than proIL-37b. These results demonstrate that plants have great potential of being a green bioreactor for low-cost, large-scale production of biologically active IL-37.

Transient plant production of Salmonella Typhimurium diagnostic antibodies

Salmonella Typhimurium is one of the most important zoonotic pathogens worldwide and a major cause of economic losses in the pig production chain. The emergence of multi-drug resistant strains over the past years has led to considerations about an enhanced surveillance of bacterial food contamination. Currently, ELISA is the method of choice for high throughput identification of S. Typhimurium. The sensitivity and specificity of this assay might be improved by application of new diagnostic antibodies. We focused on plant-based expression of candidate diagnostic TM43-E10 antibodies discovered using as antigen the S. Typhimurium OmpD protein. The scFv-TM43-E10 and scFv-Fc-TM43-E10 antibody derivatives have been successfully produced in N. benthamiana using a deconstructed movement-deficient PVX vector supplemented with the γb silencing suppressor from Poa semilatent virus. The plant-made antibodies showed the same antigen-binding specificity as that of the microbial/mammalian cell-produced counterparts and could recognize the OmpD antigen in S. Typhimurium infected plant samples.

Transient Expression of Human Cytochrome P450s 2D6 and 3A4 in Nicotiana benthamiana Provides a Possibility for Rapid Substrate Testing and Production of Novel Compounds

Employment of transient expression of foreign genes for bioconversion of pharmaceutically valuable low-molecular-weight compounds, including plant secondary metabolites, is an enticing trend still scantily explored in plant biotechnology. In the present work, an efficient protocol for rapid assessment of synthetic and plant-derived metabolites as potential substrates for human P450s (CYP2D6 and CYP3A4) via Agrobacterium-mediated transient expression in Nicotiana benthamiana is put forth. Animal P450s with broad substrate specificity are promising candidates for transformation of diverse metabolites. The efficiency of P450s in heterologous surroundings is not always satisfactory and depends on the availability of an associated electron-transfer enzyme. Plants represent an attractive assortment of prospective hosts for foreign P450s expression. The optimal composition of genetic blocks providing the highest transient expression efficiency is designed, an effective substrate administration scheme is validated, and biological activity of the investigated P450s against loratadine and several indole alkaloids with different molecular scaffold structures is tested. A novel indole alkaloid, 11-hydroxycorynanthine, is isolated from N. benthamiana plants transiently expressing CYP2D6 and supplemented with corynanthine, and its structure was elucidated. The proposed technique might be of value in realization of combinatorial biosynthesis concept comprising the junction of heterologous enzymes and substrates in different metabolic surroundings.

Development of a new tobamovirus-based viral vector for protein expression in plants

Plants are becoming an interesting alternative system for the heterologous production of pharmaceutical proteins, providing a more scalable, cost-effective, and biologically safer option than the current expression systems. The development of plant virus expression vectors has allowed rapid and high-level transient expression of recombinant genes, and, in turn, provided an attractive plant-based production platform. Here we report the development of vectors based on the tobamovirus Pepper mild mottle virus (PMMoV) to be used in transient expression of foreign genes. In this PMMoV vector, a middle part of the viral coat protein gene was replaced by the green fluorescent protein (GFP) gene, and this recombinant genome was assembled in a binary vector suitable for plant agroinoculation. The accumulation of GFP was evaluated by observation of green fluorescent signals under UV light and by western blotting. Furthermore, by using this vector, the multiepitope gene for chikungunya virus was successfully expressed and confirmed by western blotting. This PMMoV-based vector represents an alternative system for a high-level production of heterologous protein in plants.

Assessment of the immunogenicity of residual host cell protein impurities of OsrHSA

Human serum albumin (HSA) is the most abundant protein in human plasma and is widely used at high doses for treating various diseases. Recombinant HSA is an alternative approach to plasma-derived HSA, providing increased safety and an unlimited supply. However, the safety of the residual host cell proteins (HCPs) co-purified with Oryza sativa HSA (OsrHSA) remains to be determined. An animal system was used to assess the immunogenicity of OsrHSA and its residual HCPs. Low immunogenicity and immunotoxicity of the residual HCPs at a dose of 25 μg/kg, equivalent to 25 times the clinical dosage of HSA, were observed. An anti-drug-antibody (ADA) analysis revealed that anti-HSA, anti-OsrHSA or anti-HCP antibodies developed with a low frequency in pHSA and OsrHSA treatments, but the titers were as low as 1.0–2.0. Furthermore, the titer and the incidence of the specific antibodies were not significantly different between the pHSA and OsrHSA groups, indicating that OsrHSA presents similar immunogenicity to that of pHSA. More importantly, no cytokines were stimulated after the administration of OsrHSA and the residual HCPs, suggesting that there was no risk of a cytokine storm. These results demonstrated that the residual HCPs from OsrHSA have low immunogenicity, indicating that the rice endosperm is one of the best hosts for plant molecular pharming.

A synthetic biology approach for consistent production of plant-made recombinant polyclonal antibodies against snake venom toxins

Antivenoms developed from the plasma of hyperimmunized animals are the only effective treatment available against snakebite envenomation but shortage of supply contributes to the high morbidity and mortality toll of this tropical disease. We describe a synthetic biology approach to affordable and cost-effective antivenom production based on plant-made recombinant polyclonal antibodies (termed pluribodies). The strategy takes advantage of virus superinfection exclusion to induce the formation of somatic expression mosaics in agroinfiltrated plants, which enables the expression of complex antibody repertoires in a highly reproducible manner. Pluribodies developed using toxin-binding genetic information captured from peripheral blood lymphocytes of hyperimmunized camels recapitulated the overall binding activity of the immune response. Furthermore, an improved plant-made antivenom (plantivenom) was formulated using an in vitro selected pluribody against Bothrops asper snake venom toxins and has been shown to neutralize a wide range of toxin activities and provide protection against lethal venom doses in mice.

Platforms for Plant-Based Protein Production

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

Platforms for Recombinant Therapeutic Glycoprotein Production

The majority of FDA-approved biology-derived products are recombinant glycoproteins. These proteins have been used for the treatment of several diseases, with numerous products currently approved for clinical use. The choice of the expression system is a key step toward a successful functional protein production, since glycosylation influences yield, pharmacokinetics, biological activity, and immunogenicity. This chapter covers the general aspects of therapeutic recombinant glycoproteins and the platforms that are being employed for their production.

Synthetic gene design-The rationale for codon optimization and implications for molecular pharming in plants

Degeneracy in the genetic code allows multiple codon sequences to encode the same protein. Codon usage bias in genes is the term given to the preferred use of particular synonymous codons. Synonymous codon substitutions had been regarded as "silent" as the primary structure of the protein was not affected; however, it is now accepted that synonymous substitutions can have a significant effect on heterologous protein expression. Codon optimization, the process of altering codons within the gene sequence to improve recombinant protein expression, has become widely practised. Multiple inter-linked factors affecting protein expression need to be taken into consideration when optimizing a gene sequence. Over the years, various computer programmes have been developed to aid in the gene sequence optimization process. However, as the rulebook for altering codon usage to affect protein expression is still not completely understood, it is difficult to predict which strategy, if any, will design the "optimal" gene sequence. In this review, codon usage bias and factors affecting codon selection will be discussed and the evidence for codon optimization impact will be reviewed for recombinant protein expression using plants as a case study. These developments will be relevant to all recombinant expression systems; however, molecular pharming in plants is an area which has consistently encountered difficulties with low levels of recombinant protein expression, and should benefit from an evidence based rational approach to synthetic gene design. Biotechnol. Bioeng. 2017;114: 492-502. © 2016 Wiley Periodicals, Inc.

Challenges and perspectives in commercializing plastid transformation technology

Plastid transformation has emerged as an alternative platform to generate transgenic plants. Attractive features of this technology include specific integration of transgenes-either individually or as operons-into the plastid genome through homologous recombination, the potential for high-level protein expression, and transgene containment because of the maternal inheritance of plastids. Several issues associated with nuclear transformation such as gene silencing, variable gene expression due to the Mendelian laws of inheritance, and epigenetic regulation have not been observed in the plastid genome. Plastid transformation has been successfully used for the production of therapeutics, vaccines, antigens, and commercial enzymes, and for engineering various agronomic traits including resistance to biotic and abiotic stresses. However, these demonstrations have usually focused on model systems such as tobacco, and the technology per se has not yet reached the market. Technical factors limiting this technology include the lack of efficient protocols for the transformation of cereals, poor transgene expression in non-green plastids, a limited number of selection markers, and the lengthy procedures required to recover fully segregated plants. This article discusses the technology of transforming the plastid genome, the positive and negative features compared with nuclear transformation, and the current challenges that need to be addressed for successful commercialization.

The Last Ten Years of Advancements in Plant-Derived Recombinant Vaccines against Hepatitis B

Disease prevention through vaccination is considered to be the greatest contribution to public health over the past century. Every year more than 100 million children are vaccinated with the standard World Health Organization (WHO)-recommended vaccines including hepatitis B (HepB). HepB is the most serious type of liver infection caused by the hepatitis B virus (HBV), however, it can be prevented by currently available recombinant vaccine, which has an excellent record of safety and effectiveness. To date, recombinant vaccines are produced in many systems of bacteria, yeast, insect, and mammalian and plant cells. Among these platforms, the use of plant cells has received considerable attention in terms of intrinsic safety, scalability, and appropriate modification of target proteins. Research groups worldwide have attempted to develop more efficacious plant-derived vaccines for over 30 diseases, most frequently HepB and influenza. More inspiring, approximately 12 plant-made antigens have already been tested in clinical trials, with successful outcomes. In this study, the latest information from the last 10 years on plant-derived antigens, especially hepatitis B surface antigen, approaches are reviewed and breakthroughs regarding the weak points are also discussed.

Tackling Unwanted Proteolysis in Plant Production Hosts Used for Molecular Farming

Although the field of molecular farming has significantly matured over the last years, some obstacles still need to be resolved. A major limiting factor for a broader application of plant hosts for the production of valuable recombinant proteins is the low yield of intact recombinant proteins. These low yields are at least in part due to the action of endogenous plant proteases on the foreign recombinant proteins. This mini review will present the current knowledge of the proteolytic enzymes involved in the degradation of different target proteins and strategies that are applied to suppress undesirable proteolytic activities in order to safeguard recombinant proteins during the production process.

A Chimeric Affinity Tag for Efficient Expression and Chromatographic Purification of Heterologous Proteins from Plants

The use of plants as expression hosts for recombinant proteins is an increasingly attractive option for the production of complex and challenging biopharmaceuticals. Tools are needed at present to marry recent developments in high-yielding gene vectors for heterologous expression with routine protein purification techniques. In this study, we designed the Cysta-tag, a new purification tag for immobilized metal affinity chromatography (IMAC) of plant-made proteins based on the protein-stabilizing fusion partner SlCYS8. We show that the Cysta-tag may be used to readily purify proteins under native conditions, and then be removed enzymatically to isolate the protein of interest. We also show that commonly used protease recognition sites for linking purification tags are differentially stable in leaves of the commonly used expression host Nicotiana benthamiana, with those linkers susceptible to cysteine proteases being less stable then serine protease-cleavable linkers. As an example, we describe a Cysta-tag experimental scheme for the one-step purification of a clinically useful protein, human α1-antitrypsin, transiently expressed in N. benthamiana. With potential applicability to the variety of chromatography formats commercially available for IMAC-based protein purification, the Cysta-tag provides a convenient means for the efficient and cost-effective purification of recombinant proteins from plant tissues.

Recombinant IgA production for mucosal passive immunization, advancing beyond the hurdles

Vaccination is a successful strategy to proactively develop immunity to a certain pathogen, but most vaccines fail to trigger a specific immune response at the mucosal surfaces, which are the first port of entry for infectious agents. At the mucosal surfaces, the predominant immunoglobulin is secretory IgA (SIgA) that specifically neutralizes viruses and prevents bacterial colonization. Mucosal passive immunization, i.e. the application of pathogen-specific SIgAs at the mucosae, can be an effective alternative to achieve mucosal protection. However, this approach is not straightforward, mainly because SIgAs are difficult to obtain from convalescent sources, while recombinant SIgA production is challenging due to its complex structure. This review provides an overview of manufacturing difficulties presented by the unique structural diversity of SIgAs, and the innovative solutions being explored for SIgA production in mammalian and plant expression systems.

Overcoming low yields of plant-made antibodies by a protein engineering approach

The commercial development of plant-based antibody production platforms is often limited by low and variable yields, but little is known about the factors that affect antibody accumulation during and after translation. Here, we present a strategy to identify yield-limiting regions in the transcript and protein. We exchanged variable heavy chain (VH) domain sequences between two human antibodies at structurally conserved positions, thus creating ten chimeric VH domains containing sequences from M12 (∼1000 μg/g leaf fresh weight [FW]) and 4E10 (∼100 μg/g FW). After transient expression in Nicotiana benthamiana leaves, we measured mRNA and protein levels by quantitative real-time PCR and surface plasmon resonance spectroscopy, respectively. Transcript levels were similar for all constructs, but antibody levels ranged from ∼250 μg/g to over 2000 μg/g FW. Analysis of the expression levels showed that: i) 4E10 yields were only marginally increased by suppression of post-transcriptional gene silencing; ii) the CDR3 of 4E10 contains a protease site; and iii) a bipartite, yield-limiting region exists in the CDR2/CDR3. Our findings highlight the strong impact of cotranslational and posttranslational events on antibody yields and show that protein engineering is a powerful tool that can be used to overcome the remaining limitations affecting antibody production in plants.

Potential of plants to produce recombinant protein products

Plants have great potential as photosynthetic factories to produce pharmaceutically important and commercially valuable biomedicines and industrial proteins at low cost. The U.S. Food and Drug Administration (U.S. FDA) has approved the drug Elelyso (taliglucerase alfa) produced by carrot cells for treatment of type 1 Gaucher's disease in 2012. The commercial potential of biomedicines produced by molecular farming has dramatically improved due to the success of an experimental drug called ZMapp, which has immunological activity in Ebola patients. A cocktail of three monoclonal antibodies was produced in tobacco (Nicotiana benthamiana) plants (Chen and Davis 2016). At present, very few drugs made by this technology have been approved by worldwide authorities such as the U.S. FDA. However, plants have been proposed as a novel paradigm for commercial production of proteins over the next decade. In recent years, leading researchers on molecular farming have given more priority to the area of animal-free therapeutic proteins such as parenteral and oral vaccines. Although plant-based platforms have considerable advantages over traditional systems such as bacterial and animal systems, there are several obstacles to commercial-scale production, especially with regards to improving the quality and quantity of plant-produced biologics and industrial materials. One of the biggest barriers to commercialization of this technology is the intense scrutiny of these new plant varieties by regulatory agencies and the public as well as the high costs associated with their regulatory approval.

Plants as Factories for Human Pharmaceuticals: Applications and Challenges

Plant molecular farming (PMF), defined as the practice of using plants to produce human therapeutic proteins, has received worldwide interest. PMF has grown and advanced considerably over the past two decades. A number of therapeutic proteins have been produced in plants, some of which have been through pre-clinical or clinical trials and are close to commercialization. Plants have the potential to mass-produce pharmaceutical products with less cost than traditional methods. Tobacco-derived antibodies have been tested and used to combat the Ebola outbreak in Africa. Genetically engineered immunoadhesin (DPP4-Fc) produced in green plants has been shown to be able to bind to MERS-CoV (Middle East Respiratory Syndrome), preventing the virus from infecting lung cells. Biosafety concerns (such as pollen contamination and immunogenicity of plant-specific glycans) and costly downstream extraction and purification requirements, however, have hampered PMF production from moving from the laboratory to industrial application. In this review, the challenges and opportunities of PMF are discussed. Topics addressed include; transformation and expression systems, plant bioreactors, safety concerns, and various opportunities to produce topical applications and health supplements.

Efficient Secretion of Recombinant Proteins from Rice Suspension-Cultured Cells Modulated by the Choice of Signal Peptide

Plant-based expression systems have emerged as a competitive platform in the large-scale production of recombinant proteins. By adding a signal peptide, αAmy3sp, the desired recombinant proteins can be secreted outside transgenic rice cells, making them easy to harvest. In this work, to improve the secretion efficiency of recombinant proteins in rice expression systems, various signal peptides including αAmy3sp, CIN1sp, and 33KDsp have been fused to the N-terminus of green fluorescent protein (GFP) and introduced into rice cells to explore the efficiency of secretion of foreign proteins. 33KDsp had better efficiency than αAmy3sp and CIN1sp for the secretion of GFP from calli and suspension-cultured cells. 33KDsp was further applied for the secretion of mouse granulocyte-macrophage colony-stimulating factor (mGM-CSF) from transgenic rice suspension-cultured cells; approximately 76%–92% of total rice-derived mGM-CSF (rmGM-CSF) was detected in the culture medium. The rmGM-CSF was bioactive and could stimulate the proliferation of a murine myeloblastic leukemia cell line, NSF-60. The extracellular yield of rmGM-CSF reached 31.7 mg/L. Our study indicates that 33KDsp is better at promoting the secretion of recombinant proteins in rice suspension-cultured cell systems than the commonly used αAmy3sp.

Assembly and Purification of Polyomavirus-Like Particles from Plants

Polyomaviruses are small DNA viruses that have a history of use in biotechnology. The capsids of a number of species have been developed into experimental prophylactic and therapeutic virus-like particle (VLP) vaccines. In order to explore plants as a host for the expression and purification of polyomavirus-like particles, we have transiently expressed the major capsid protein, VP1, in Nicotiana benthamiana leaves. Deletion of a polybasic motif from the N-terminal region of VP1 resulted in increased expression as well as reduced necrosis of leaf tissue, which was associated with differences in subcellular localisation and reduced DNA binding by the deletion variant (ΔVP1). Self-assembled VLPs were recovered from tissue expressing both wild-type VP1 and ΔVP1 by density gradient ultracentrifugation. VLPs composed of ΔVP1 were more homogenous than wtVPLs and, unlike the latter, did not encapsidate nucleic acid. Such homogenous, empty VLPs are of great interest in biotechnology and nanotechnology. In addition, we show that both MPyV VLP variants assembled in plants can be produced with encapsidated foreign protein. Thus, this study demonstrates the utility of plant-based expression of polyomavirus-like particles and the suitability of this host for further developments in polyomavirus-based technologies.

Moss-made pharmaceuticals: from bench to bedside

Over the past two decades, the moss Physcomitrella patens has been developed from scratch to a model species in basic research and in biotechnology. A fully sequenced genome, outstanding possibilities for precise genome-engineering via homologous recombination (knockout moss), a certified GMP production in moss bioreactors, successful upscaling to 500 L wave reactors, excellent homogeneity of protein glycosylation, remarkable batch-to-batch stability and a safe cryopreservation for master cell banking are some of the key features of the moss system. Several human proteins are being produced in this system as potential biopharmaceuticals. Among the products are tumour-directed monoclonal antibodies with enhanced antibody-dependent cytotoxicity (ADCC), vascular endothelial growth factor (VEGF), complement factor H (FH), keratinocyte growth factor (FGF7/KGF), epidermal growth factor (EGF), hepatocyte growth factor (HGF), asialo-erythropoietin (asialo-EPO, AEPO), alpha-galactosidase (aGal) and beta-glucocerebrosidase (GBA). Further, an Env-derived multi-epitope HIV protein as a candidate vaccine was produced, and first steps for a metabolic engineering of P. patens have been made. Some of the recombinant biopharmaceuticals from moss bioreactors are not only similar to those produced in mammalian systems such as CHO cells, but are of superior quality (biobetters). The first moss-made pharmaceutical, aGal to treat Morbus Fabry, is in clinical trials.