Transgenic plants as factories for biopharmaceuticals

Fabricated Elastin

The mechanical stability, elasticity, inherent bioactivity, and self-assembly properties of elastin make it a highly attractive candidate for the fabrication of versatile biomaterials. The ability to engineer specific peptide sequences derived from elastin allows the precise control of these physicochemical and organizational characteristics, and further broadens the diversity of elastin-based applications. Elastin and elastin-like peptides can also be modified or blended with other natural or synthetic moieties, including peptides, proteins, polysaccharides, and polymers, to augment existing capabilities or confer additional architectural and biofunctional features to compositionally pure materials. Elastin and elastin-based composites have been subjected to diverse fabrication processes, including heating, electrospinning, wet spinning, solvent casting, freeze-drying, and cross-linking, for the manufacture of particles, fibers, gels, tubes, sheets and films. The resulting materials can be tailored to possess specific strength, elasticity, morphology, topography, porosity, wettability, surface charge, and bioactivity. This extraordinary tunability of elastin-based constructs enables their use in a range of biomedical and tissue engineering applications such as targeted drug delivery, cell encapsulation, vascular repair, nerve regeneration, wound healing, and dermal, cartilage, bone, and dental replacement.

Toxins and derivatives in molecular pharmaceutics: Drug delivery and targeted therapy

Protein and peptide toxins offer an invaluable source for the development of actively targeted drug delivery systems. They avidly bind to a variety of cognate receptors, some of which are expressed or even up-regulated in diseased tissues and biological barriers. Protein and peptide toxins or their derivatives can act as ligands to facilitate tissue- or organ-specific accumulation of therapeutics. Some toxins have evolved from a relatively small number of structural frameworks that are particularly suitable for addressing the crucial issues of potency and stability, making them an instrumental source of leads and templates for targeted therapy. The focus of this review is on protein and peptide toxins for the development of targeted drug delivery systems and molecular therapies. We summarize disease- and biological barrier-related toxin receptors, as well as targeted drug delivery strategies inspired by those receptors. The design of new therapeutics based on protein and peptide toxins is also discussed.

The Corn Smut ('Huitlacoche') as a New Platform for Oral Vaccines

The development of new alternative platforms for subunit vaccine production is a priority in the biomedical field. In this study, Ustilago maydis, the causal agent of common corn smut or ‘huitlacoche’has been genetically engineered to assess expression and immunogenicity of the B subunit of the cholera toxin (CTB), a relevant immunomodulatory agent in vaccinology. An oligomeric CTB recombinant protein was expressed in corn smut galls at levels of up to 1.3 mg g-1 dry weight (0.8% of the total soluble protein). Mice orally immunized with ‘huitlacoche’-derived CTB showed significant humoral responses that were well-correlated with protection against challenge with the cholera toxin (CT). These findings demonstrate the feasibility of using edible corn smut as a safe, effective, and low-cost platform for production and delivery of a subunit oral vaccine. The implications of this platform in the area of molecular pharming are discussed.

Expression of H3N2 nucleoprotein in maize seeds and immunogenicity in mice

Oral administration of maize-expressed H3N2 nucleoprotein induced antibody responses in mice showing the immunogenicity of plant-derived antigen and its potential to be utilized as a universal flu vaccine. Influenza A viruses cause influenza epidemics that are devastating to humans and livestock. The vaccine for influenza needs to be reformulated every year to match the circulating strains due to virus mutation. Influenza virus nucleoprotein (NP) is a multifunctional RNA-binding protein that is highly conserved among strains, making it a potential candidate for a universal vaccine. In this study, the NP gene of H3N2 swine origin influenza virus was expressed in maize endosperm. Twelve transgenic maize lines were generated and analyzed for recombinant NP (rNP) expression. Transcript analysis showed the main accumulation of rNP in seed. Protein level of rNP in T1 transgenic maize seeds ranged from 8.0 to 35 µg of NP/g of corn seed. The level increased up to 70 µg of NP/g in T3 seeds. A mouse study was performed to test the immunogenicity of one line of maize-derived rNP (MNP). Mice were immunized with MNP in a prime-boost design. Oral gavage administration showed that a humoral immune response was elicited in the mice treated with MNP indicating the immunogenicity of MNP. NP-specific antibody responses in the MNP group showed comparable antibody titer with the groups receiving positive controls such as Vero cell-derived NP (VNP) or alphavirus replicon particle-derived NP (ANP). Cytokine analysis showed antigen-specific stimulation of IL-4 cytokine elicited in splenocytes from mice treated with MNP further confirming a TH2 humoral immune response induced by MNP administration.

Expression of recombinant vaccines and antibodies in plants

Plants are able to perform post-translational maturations of therapeutic proteins required for their functional biological activity and suitable in vivo pharmacokinetics. Plants can be a low-cost, large-scale production platform of recombinant biopharmaceutical proteins such as vaccines and antibodies. Plants, however, lack mechanisms of processing authentic human N-glycosylation, which imposes a major limitation in their use as an expression system for therapeutic glycoproducts. Efforts have been made to circumvent plant-specific N-glycosylation, as well as to supplement the plant's endogenous system with human glycosyltransferases for non-immunogenic and humanized N-glycan production. Herein we review studies on the potential of plants to serve as production systems for therapeutic and prophylactic biopharmaceuticals. We have especially focused on recombinant vaccines and antibodies and new expression strategies to overcome the existing problems associated with their production in plants.

Edible Vaccines

Vaccines are considered primary tools for health intervention in both humans and animals. Vaccines can be used more widely, especially in developing countries, if their cost of production can be reduced and they can be preserved without refrigeration. In developing countries certain limitations, like vaccine affordability, the need for “cold chains” from the producer to the site of use of the vaccine, and the dependence on injection, are barriers to health care services. Plant-derived vaccines do not face such limitations. Research under way is dedicated to solving these limitations by finding ways to produce oral (edible) vaccines from transgenic plants. Plant-derived vaccines offer increased safety, envisage low-cost programs for mass vaccination, and propose a wider use of vaccination for veterinary use.

Deletion of plant-specific sugar residues in plant N-glycans by repression of GDP-D-mannose 4,6-dehydratase and β-1,2-xylosyltransferase genes

Production of pharmaceutical glycoproteins, such as therapeutic antibodies and cytokines, in plants has many advantages in safety and reduced costs. However, plant-made glycoproteins have N-glycans with plant-specific sugar residues (core β-1,2-xylose and α-1,3-fucose) and a Lewis a (Le(a)) epitope, Galβ(1-3)[Fucα(1-4)]GlcNAc. Because it is likely that these sugar residues and glycan structures are immunogenic, many attempts have been made to delete them. Previously, we reported the simultaneous deletion of the plant-specific core α-1,3-fucose and α-1,4-fucose residues in Le(a) epitopes by repressing the GDP-D-mannose 4,6-dehydratase (GMD) gene, which is associated with GDP-L-fucose biosynthesis, in Nicotiana benthamiana plants (rGMD plants, renamed to ΔGMD plants) (Matsuo and Matsumura, Plant Biotechnol. J., 9, 264-281, 2011). In the present study, we generated a core β-1,2-xylose residue-repressed transgenic N. benthamiana plant by co-suppression of β-1,2-xylosyltransferase (ΔXylT plant). By crossing ΔGMD and ΔXylT plants, we successfully generated plants in which plant-specific sugar residues were repressed (ΔGMDΔXylT plants). The proportion of N-glycans with deleted plant-specific sugar residues found in total soluble protein from ΔGMDΔXylT plants increased by 82.41%. Recombinant mouse granulocyte/macrophage-colony stimulating factor (mGM-CSF) and human monoclonal immunoglobulin G (hIgG) harboring N-glycans with deleted plant-specific sugar residues were successfully produced in ΔGMDΔXylT plants. Simultaneous repression of the GMD and XylT genes in N. benthamiana is thus very useful for deleting plant-specific sugar residues.

Production of prostaglandins in transgenic Arabidopsis thaliana

Plants do not naturally produce the very-long-chain polyunsaturated fatty acids that are the precursors of prostaglandins, but in previous studies Arabidopsis thaliana had been transformed sequentially with genes encoding a Δ(9)-elongase and a Δ(8)-desaturase to produce dihomo-γ-linolenic acid (DGLA) and eicosatetraenoic acid (ETA), and subsequently with a gene encoding a Δ(5)-desaturase to produce arachidonic acid (AA) and eicosapentaenoic acid (EPA). Transformation of A. thaliana with the first two genes consolidated on a single binary vector yielded transformants producing high levels of DGLA, and these plants were further transformed with mouse prostaglandin H synthase (PGH) genes to produce prostaglandins. Mouse PGHS-1 and PGHS-2 cDNAs were amplified for expression as three isoforms: PGHS-1 (complete coding sequence with signal peptide), PGHS-1-Ma (mature PGHS-1 sequence, without signal peptide) and PGHS-2 (complete coding sequence with signal peptide). PGHS-1 transformants showed the highest activity, followed by PGHS-2 transformants, whereas removal of the signal peptide resulted in almost complete loss of PGHS-1 activity. In order to produce a physiologically active prostaglandin, the Trypanosoma brucei prostaglandin F synthase gene was combined with the mouse PGHS-1 gene and the Mortierella alpina Δ(5)-desaturase on a binary vector. Transformation of DGLA-producing A. thaliana with this construct yielded transformants that successfully produced prostaglandin F.

N-glycosylation modification of plant-derived virus-like particles: an application in vaccines

Plants have been developed as an alternative system to mammalian cells for production of recombinant prophylactic or therapeutic proteins for human and animal use. Effective plant expression systems for recombinant proteins have been established with the optimal combination of gene expression regulatory elements and control of posttranslational processing of recombinant glycoproteins. In plant, virus-like particles (VLPs), viral “empty shells” which maintain the same structural characteristics of virions but are genome-free, are considered extremely promising as vaccine platforms and therapeutic delivery systems. Unlike microbial fermentation, plants are capable of carrying out N-glycosylation as a posttranslational modification of glycoproteins. Recent advances in the glycoengineering in plant allow human-like glycomodification and optimization of desired glycan structures for enhancing safety and functionality of recombinant pharmaceutical glycoproteins. In this review, the current plant-derived VLP approaches are focused, and N-glycosylation and its in planta modifications are discussed.

Production of Lupin Acid Phosphatase in Transgenic Rice for Use as a Phytate-hydrolyzing Enzyme in Animal Feed

The acid phosphatase gene from lupin was expressed in transgenic rice plants under the control of the maize ubiquitin promoter or rice chlorophyll a/b binding protein (Cab) promoter. Transgenic rice leaves exhibited up to an 18-fold increase in phytate-hydrolyzing activity. Based on the phytate-hydrolyzing activity at pH 5.5, more than 85% this activity was retained after heat-treatment at 80 degrees C for 15 min, and the heterologous enzyme in leaf sections and leaf extracts was relatively stable during storage. A distinct increase in released phosphate was observed when the heterologous enzyme was mixed with the feed extract. These results suggest that the heterologous enzyme in rice plants may maintain its desired characteristics as a phytate-hydrolyzing enzyme when added to animal feed.

The use of versatile plant antimicrobial peptides in agribusiness and human health

Plant immune responses involve a wide diversity of physiological reactions that are induced by the recognition of pathogens, such as hypersensitive responses, cell wall modifications, and the synthesis of antimicrobial molecules including antimicrobial peptides (AMPs). These proteinaceous molecules have been widely studied, presenting peculiar characteristics such as conserved domains and a conserved disulfide bond pattern. Currently, many AMP classes with diverse modes of action are known, having been isolated from a large number of organisms. Plant AMPs comprise an interesting source of studies nowadays, and among these there are reports of different classes, including defensins, albumins, cyclotides, snakins and several others. These peptides have been widely used in works that pursue human disease control, including nosocomial infections, as well as for agricultural purposes. In this context, this review will focus on the relevance of the structural-function relations of AMPs derived from plants and their proper use in applications for human health and agribusiness.

Production of functional native human interleukin-2 in tobacco chloroplasts

Interleukin-2 (IL-2) is an important T lymphocyte-derived cytokine in the mammalian immune system. Non-native, recombinant IL-2 derived from Escherichia coli is used widely in both medical research and treatment of diseases. Recombinant human IL-2 gene has been expressed in plant nuclear genomes, therefore it can be spread to the environment through pollen. Furthermore, all the plant-produced IL-2 reported thus far had been attached with artificial tags or fusion proteins, which may trigger unintended immunological responses and therefore compromise its full utility as a medicine. To expand the potential of using plant chloroplasts to produce functional native human therapeutic proteins, we inserted an engineered human interleukin-2 (hIL-2)-coding gene, without any tags, into the chloroplast genome of tobacco (Nicotiana tabacum L.). Partially purified hIL-2 protein from the leaves of the transplastomic plants induced in vitro proliferation of IL-2-dependent murine T lymphocytes. Our study demonstrates that plant chloroplasts can serve as a bio-factory for production of an active native human interleukin in a self-contained and therefore environmentally safe manner.

Genes involved in the endoplasmic reticulum N-glycosylation pathway of the red microalga Porphyridium sp.: a bioinformatic study

N-glycosylation is one of the most important post-translational modifications that influence protein polymorphism, including protein structures and their functions. Although this important biological process has been extensively studied in mammals, only limited knowledge exists regarding glycosylation in algae. The current research is focused on the red microalga Porphyridium sp., which is a potentially valuable source for various applications, such as skin therapy, food, and pharmaceuticals. The enzymes involved in the biosynthesis and processing of N-glycans remain undefined in this species, and the mechanism(s) of their genetic regulation is completely unknown. In this study, we describe our pioneering attempt to understand the endoplasmic reticulum N-Glycosylation pathway in Porphyridium sp., using a bioinformatic approach. Homology searches, based on sequence similarities with genes encoding proteins involved in the ER N-glycosylation pathway (including their conserved parts) were conducted using the TBLASTN function on the algae DNA scaffold contigs database. This approach led to the identification of 24 encoded-genes implicated with the ER N-glycosylation pathway in Porphyridium sp. Homologs were found for almost all known N-glycosylation protein sequences in the ER pathway of Porphyridium sp.; thus, suggesting that the ER-pathway is conserved; as it is in other organisms (animals, plants, yeasts, etc.).

Expression of recombinant antibody (single chain antibody fragment) in transgenic plant Nicotiana tabacum cv. Xanthi

Plants offer an alternative inexpensive and convenient technology for large scale production of recombinant proteins especially recombinant antibodies (plantibodies). In this paper, we describe the expression of a model single chain antibody fragment (B6scFv) in transgenic tobacco. Four different gene constructs of B6scFv with different target signals for expression in different compartments of a tobacco plant cell with and without endoplasmic reticulum (ER) retention signal were used. Agrobacterium mediated plant transformation of B6scFv gene was performed with tobacco leaf explants and the gene in regenerated plants was detected using histochemical GUS assay and PCR. The expression of B6scFv gene was detected by western blotting and the recombinant protein was purified from putative transgenic tobacco plants using metal affinity chromatography. The expression level of recombinant protein was determined by indirect enzyme-linked immunosorbent assay. The highest accumulation of protein was found up to 3.28 % of the total soluble protein (TSP) in plants expressing B6scFv 1003 targeted to the ER, and subsequently expression of 2.9 % of TSP in plants expressing B6scFv 1004 (with target to apoplast with ER retention signal). In contrast, lower expression of 0.78 and 0.58 % of TSP was found in plants expressing antibody fragment in cytosol and apoplast, without ER retention signal. The described method/system could be used in the future for diverse applications including expression of other recombinant molecules in plants for immunomodulation, obtaining pathogen resistance against plant pathogens, altering metabolic pathways and also for the expression of different antibodies of therapeutic and diagnostic uses.

Secretion of polyhydroxybutyrate in Escherichia coli using a synthetic biological engineering approach

BACKGROUND

Polyhydroxyalkanoates (PHAs) are a group of biodegradable plastics that are produced by a wide variety of microorganisms, mainly as a storage intermediate for energy and carbon. Polyhydroxybutyrate (PHB) is a short-chain-length PHA with interesting chemical and physical properties. Large scale production of PHB is not wide-spread mainly due to the downstream processing of bacterial cultures to extract the PHB. Secretion of PHB from Escherichia coli could reduce downstream processing costs. PHB are non-proteinaceous polymers, hence cannot be directly targeted for secretion. Phasin, PhaP1, is a low molecular weight protein that binds to PHB, reducing PHB granule size. In this study PHB is indirectly secreted with PhaP1 from E. coli via type I secretion using HlyA signal peptides.

RESULTS

This study demonstrated the successful secretion of phasin and phasin bound PHB outside of the cell and into the culture medium. The secretion of PHB was initiated between 24 and 48 h after induction. After 48 h of culturing, 36% of the total PHB produced in the secreting strain was collected in the secreted fraction and 64% remained in the internal fraction. To further support the findings of this study, the PHB secretion phenomenon was observed using SEM.

CONCLUSIONS

From this study, the ability to use type I secretion to: 1) secrete phasin and 2) successfully secrete PHB has been shown.

Plant-derived pharmaceuticals for the developing world

Plant-produced vaccines and therapeutic agents offer enormous potential for providing relief to developing countries by reducing the incidence of infant mortality caused by infectious diseases. Vaccines derived from plants have been demonstrated to effectively elicit an immune response. Biopharmaceuticals produced in plants are inexpensive to produce, require fewer expensive purification steps, and can be stored at ambient temperatures for prolonged periods of time. As a result, plant-produced biopharmaceuticals have the potential to be more accessible to the rural poor. This review describes current progress with respect to plant-produced biopharmaceuticals, with a particular emphasis on those that target developing countries. Specific emphasis is given to recent research on the production of plant-produced vaccines toward human immunodeficiency virus, malaria, tuberculosis, hepatitis B virus, Ebola virus, human papillomavirus, rabies virus and common diarrheal diseases. Production platforms used to express vaccines in plants, including nuclear and chloroplast transformation, and the use of viral expression vectors, are described in this review. The review concludes by outlining the next steps for plant-produced vaccines to achieve their goal of providing safe, efficacious and inexpensive vaccines to the developing world.

Expression of verocytotoxic Escherichia coli antigens in tobacco seeds and evaluation of gut immunity after oral administration in mouse model

Verocytotoxic Escherichia (E.) coli strains are responsible for swine oedema disease, which is an enterotoxaemia that causes economic losses in the pig industry. The production of a vaccine for oral administration in transgenic seeds could be an efficient system to stimulate local immunity. This study was conducted to transform tobacco plants for the seed-specific expression of antigenic proteins from a porcine verocytotoxic E. coli strain. Parameters related to an immunological response and possible adverse effects on the oral administration of obtained tobacco seeds were evaluated in a mouse model. Tobacco was transformed via Agrobacteium tumefaciens with chimeric constructs containing structural parts of the major subunit FedA of the F18 adhesive fimbriae and VT2e B-subunit genes under control of a seed specific GLOB promoter. We showed that the foreign Vt2e-B and F18 genes were stably accumulated in storage tissue by the immunostaining method. In addition, Balb-C mice receiving transgenic tobacco seeds via the oral route showed a significant increase in IgA-positive plasma cell presence in tunica propria when compared to the control group with no observed adverse effects. Our findings encourage future studies focusing on swine for evaluation of the protective effects of transformed tobacco seeds against E. coli infection.

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.

Expression systems and species used for transgenic animal bioreactors

Transgenic animal bioreactors can produce therapeutic proteins with high value for pharmaceutical use. In this paper, we compared different systems capable of producing therapeutic proteins (bacteria, mammalian cells, transgenic plants, and transgenic animals) and found that transgenic animals were potentially ideal bioreactors for the synthesis of pharmaceutical protein complexes. Compared with other transgenic animal expression systems (egg white, blood, urine, seminal plasma, and silkworm cocoon), the mammary glands of transgenic animals have enormous potential. Compared with other mammalian species (pig, goat, sheep, and cow) that are currently being studied as bioreactors, rabbits offer many advantages: high fertility, easy generation of transgenic founders and offspring, insensitivity to prion diseases, relatively high milk production, and no transmission of severe diseases to humans. Noticeably, for a small- or medium-sized facility, the rabbit system is ideal to produce up to 50 kg of protein per year, considering both economical and hygienic aspects; rabbits are attractive candidates for the mammary-gland-specific expression of recombinant proteins. We also reviewed recombinant proteins that have been produced by targeted expression in the mammary glands of rabbits and discussed the limitations of transgenic animal bioreactors.

Production of a de-novo designed antimicrobial peptide in Nicotiana benthamiana

Antimicrobial peptides are important defense compounds of higher organisms that can be used as therapeutic agents against bacterial and/or viral infections. We designed several antimicrobial peptides containing hydrophobic and positively charged clusters that are active against plant and human pathogens. Especially peptide SP1-1 is highly active with a MIC value of 0.1 μg/ml against Xanthomonas vesicatoria, Pseudomonas corrugata and Pseudomonas syringae pv syringae. However, for commercial applications high amounts of peptide are necessary. The synthetic production of peptides is still quite expensive and, depending on the physico-chemical features, difficult. Therefore we developed a plant/tobacco mosaic virus-based production system following the 'full virus vector strategy' with the viral coat protein as fusion partner for the designed antimicrobial peptide. Infection of Nicotiana benthamiana plants with such recombinant virus resulted in production of huge amounts of virus particles presenting the peptides all over their surface. After extraction of recombinant virions, peptides were released from the coat protein by chemical cleavage. A protocol for purification of the antimicrobial peptides using high resolution chromatographic methods has been established. Finally, we yielded up to 0.025 mg of peptide per g of infected leaf biomass. Mass spectrometric and NMR analysis revealed that the in planta produced peptide differs from the synthetic version only in missing of N-terminal amidation. But its antimicrobial activity was in the range of the synthetic one. Taken together, we developed a protocol for plant-based production and purification of biologically active, hydrophobic and positively charged antimicrobial peptide.

Expression systems for heterologous production of antimicrobial peptides

Antimicrobial peptides (AMPs) consist of molecules that act on the defense systems of numerous organisms toward multiple pathogens such as bacteria, fungi, parasites and viruses. These compounds have become extremely significant due to the increasing resistance of microorganisms to common antibiotics. However, the low quantity of peptides obtained from direct purification is, to date, still a remarkable bottleneck for scientific and industrial research development. Therefore, this review describes the main heterologous systems currently used for AMP production, including bacteria, fungi and plants, and also the related strategies for reaching greater functional peptide production. The main difficulties of each system are also described in order to provide some directions for AMP production. In summary, data revised here indicate that large-scale production of AMPs can be obtained using biotechnological tools, and the products may be applied in the pharmaceutical industry as well as in agribusiness.

Plant made anti-HIV microbicides--a field of opportunity

HIV remains a significant global burden and without an effective vaccine, it is crucial to develop microbicides to halt the initial transmission of the virus. Several microbicides have been researched with various levels of success. Amongst these, the broadly neutralising antibodies and peptide lectins are promising in that they can immediately act on the virus and have proven efficacious in in vitro and in vivo protection studies. For the purpose of development and access by the relevant population groups, it is crucial that these microbicides be produced at low cost. For the promising protein and peptide candidate molecules, it appears that current production systems are overburdened and expensive to establish and maintain. With recent developments in vector systems for protein expression coupled with downstream protein purification technologies, plants are rapidly gaining credibility as alternative production systems. Here we evaluate the advances made in host and vector system development for plant expression as well as the progress made in expressing HIV neutralising antibodies and peptide lectins using plant-based platforms.

Agrobacterium-mediated gene transfer in plants and biosafety considerations

Agrobacterium, the natures' genetic engineer, has been used as a vector to create transgenic plants. Agrobacterium-mediated gene transfer in plants is a highly efficient transformation process which is governed by various factors including genotype of the host plant, explant, vector, plasmid, bacterial strain, composition of culture medium, tissue damage, and temperature of co-cultivation. Agrobacterium has been successfully used to transform various economically and horticulturally important monocot and dicot species by standard tissue culture and in planta transformation techniques like floral or seedling infilteration, apical meristem transformation, and the pistil drip methods. Monocots have been comparatively difficult to transform by Agrobacterium. However, successful transformations have been reported in the last few years based on the adjustment of the parameters that govern the responses of monocots to Agrobacterium. A novel Agrobacterium transferred DNA-derived nanocomplex method has been developed which will be highly valuable for plant biology and biotechnology. Agrobacterium-mediated genetic transformation is known to be the preferred method of creating transgenic plants from a commercial and biosafety perspective. Agrobacterium-mediated gene transfer predominantly results in the integration of foreign genes at a single locus in the host plant, without associated vector backbone and is also known to produce marker free plants, which are the prerequisites for commercialization of transgenic crops. Research in Agrobacterium-mediated transformation can provide new and novel insights into the understanding of the regulatory process controlling molecular, cellular, biochemical, physiological, and developmental processes occurring during Agrobacterium-mediated transformation and also into a wide range of aspects on biological safety of transgenic crops to improve crop production to meet the demands of ever-growing world's population.

Bipartite and tripartite Cucumber mosaic virus-based vectors for producing the Acidothermus cellulolyticus endo-1,4-β-glucanase and other proteins in non-transgenic plants

BACKGROUND

Using plant viruses to produce desirable proteins in plants allows for using non-transgenic plant hosts and if necessary, the ability to make rapid changes in the virus construct for increased or modified protein product yields. The objective of this work was the development of advanced CMV-based protein production systems to produce Acidothermus cellulolyticus endo-1, 4-β-glucanase (E1) in non-transgenic plants.

RESULTS

We used two new Cucumber mosaic virus (CMV)-based vector systems for producing the green fluorescent protein (GFP) and more importantly, the Acidothermus cellulolyticus endo-1, 4-β-glucanase (E1) in non-transgenic Nicotiana benthamiana plants. These are the inducible CMVin (CMV-based inducible) and the autonomously replicating CMVar (CMV-based advanced replicating) systems. We modified a binary plasmid containing the complete CMV RNA 3 cDNA to facilitate insertion of desired sequences, and to give modifications of the subgenomic mRNA 4 leader sequence yielding several variants. Quantitative RT-PCR and immunoblot analysis showed good levels of CMV RNA and coat protein accumulation for some variants of both CMVin and CMVar. When genes for E1 or GFP were inserted in place of the CMV coat protein, both were produced in plants as shown by fluorescence (GFP) and immunoblot analysis. Enzymatic activity assays showed that active E1 was produced in plants with yields up to ~ 11 μg/g fresh weight (FW) for specific variant constructs. We also compared in vitro CMV genomic RNA reassortants, and CMV RNA 3 mutants which lacked the C' terminal 33 amino acids of the 3A movement protein in attempts to further increase E1 yield. Taken together specific variant constructs yielded up to ~21 μg/g FW of E1 in non-transgenic plants.

CONCLUSIONS

Intact, active E1 was rapidly produced in non-transgenic plants by using agroinfiltration with the CMV-based systems. This reduces the time and cost compared to that required to generate transgenic plants and still gives the comparable yields of active E1. Our modifications described here, including manipulating cloning sites for foreign gene introduction, enhance the ease of use. Also, N. benthamiana, which is particularly suitable for agroinfiltration, is a very good plant for transient protein production.

Robust preparative-scale extracellular production of hirudin in Escherichia coli and its purification and characterization

Hirudin variant III (HV3) is potentially useful in the prevention and treatment of cataracts. To prepare sufficient amounts of rHV3 for further preclinical studies, we developed an effective process for robust preparative-scale extracellular production of rHV3 in Escherichia coli. In a 7-l bioreactor, under the optimal fed-batch fermentation conditions, rHV3 was excreted into the culture supernatant and yielded up to 915 mg l(-1). Then, a four-step purification procedure was applied to the product, which included ultrafiltration, hydrophobic chromatography, anion-exchange chromatography, and preparative reversed-phase fast protein liquid chromatography (FPLC). The overall maximum recovery attained was 56 %, the purity reached at least 99 % as evaluated by HPLC analysis, the molecular weight was determined to be 7,011.10 Da by matrix-assisted laser-desorption time-of-flight mass spectrometry (MALDI-TOF/MS) analysis, and the pI was 4.46 as analyzed by isoelectric focusing. The N- and C-terminal sequence analysis confirmed the product homogeneity. The final product contained at most 10 pg of residual DNA per dose (0.2 mg) of rHV3 by high-sensitivity hybridization assay and at most 3 EU endotoxin protein/mg by limulus amebocyte lysate assay. Taken together, the rHV3 produced in multigram quantities in E. coli by this bioprocess meets the regulatory criteria for biopharmaceuticals and can be used as a drug candidate for preclinical studies.

Production of human growth hormone in transgenic rice seeds: co-introduction of RNA interference cassette for suppressing the gene expression of endogenous storage proteins

Rice seeds are potentially useful hosts for the production of pharmaceutical proteins. However, low yields of recombinant proteins have been observed in many cases because recombinant proteins compete with endogenous storage proteins. Therefore, we attempt to suppress endogenous seed storage proteins by RNA interference (RNAi) to develop rice seeds as a more efficient protein expression system. In this study, human growth hormone (hGH) was expressed in transgenic rice seeds using an endosperm-specific promoter from a 10 kDa rice prolamin gene. In addition, an RNAi cassette for reduction of endogenous storage protein expressions was inserted into the hGH expression construct. Using this system, the expression levels of 13 kDa prolamin and glutelin were effectively suppressed and hGH polypeptides accumulated to 470 μg/g dry weight at the maximum level in transgenic rice seeds. These results suggest that the suppression of endogenous protein gene expression by RNAi could be of great utility for increasing transgene products.

Overproduction of recombinant proteins in plants

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

Production of a His-tagged canecystatin in transgenic sugarcane

Transgenic plants have been widely used as expression systems of recombinant proteins in recent years because it can be an efficient alternative for the large-scale production of proteins. This is an area with great potential but is still not much explored. Indeed, this system can bring a breakthrough in the expression of any protein. The model used here as a protein factory was sugarcane, a crop of great global importance. This chapter describes the system that has been adopted in the routine production of transgenic sugarcane coupled with protein purification protocol. In this chapter, we describe production of transgenic sugarcane expressing a His-tagged cystatin under the control of the maize ubiquitin promoter. A transformed sugarcane plant presented high levels of protein expression and was selected for the purification of this protein through affinity chromatography in a nickel column. These studies demonstrate that sugarcane can be a viable expression system for recombinant protein production and that the His-tag purification strategy used to isolate the purified protein was effective.

Ubiquitin fusion expression and tissue-dependent targeting of hG-CSF in transgenic tobacco

BACKGROUND

Human granulocyte colony-stimulating factor (hG-CSF) is an important human cytokine which has been widely used in oncology and infection protection. To satisfy clinical needs, expression of recombinant hG-CSF has been studied in several organisms, including rice cell suspension culture and transient expression in tobacco leaves, but there was no published report on its expression in stably transformed plants which can serve as a more economical expression platform with potential industrial application.

RESULTS

In this study, hG-CSF expression was investigated in transgenic tobacco leaves and seeds in which the accumulation of hG-CSF could be enhanced through fusion with ubiquitin by up to 7 fold in leaves and 2 fold in seeds, leading to an accumulation level of 2.5 mg/g total soluble protein (TSP) in leaves and 1.3 mg/g TSP in seeds, relative to hG-CSF expressed without a fusion partner. Immunoblot analysis showed that ubiquitin was processed from the final protein product, and ubiquitination was up-regulated in all transgenic plants analyzed. Driven by CaMV 35S promoter and phaseolin signal peptide, hG-CSF was observed to be secreted into apoplast in leaves but deposited in protein storage vacuole (PSV) in seeds, indicating that targeting of the hG-CSF was tissue-dependent in transgenic tobacco. Bioactivity assay showed that hG-CSF expressed in both seeds and leaves was bioactive to support the proliferation of NFS-60 cells.

CONCLUSIONS

In this study, the expression of bioactive hG-CSF in transgenic plants was improved through ubiquitin fusion strategy, demonstrating that protein expression can be enhanced in both plant leaves and seeds through fusion with ubiquitin and providing a typical case of tissue-dependent expression of recombinant protein in transgenic plants.

The HSP terminator of Arabidopsis thaliana induces a high level of miraculin accumulation in transgenic tomatoes

High-level accumulation of the target recombinant protein is a significant issue in heterologous protein expression using transgenic plants. Miraculin, a taste-modifying protein, was accumulated in transgenic tomatoes using an expression cassette in which the miraculin gene was expressed by the cauliflower mosaic virus (CaMV) 35S promoter and the heat shock protein (HSP) terminator (MIR-HSP). The HSP terminator was derived from heat shock protein 18.2 in Arabidopsis thaliana . Using this HSP-containing cassette, the miraculin concentration in T0 transgenic tomato lines was 1.4-13.9% of the total soluble protein (TSP), and that in the T1 transgenic tomato line homozygous for the miraculin gene reached 17.1% of the TSP. The accumulation level of the target protein was comparable to levels observed with chloroplast transformation. The high-level accumulation of miraculin in T0 transgenic tomato lines achieved by the HSP terminator was maintained in the successive T1 generation, demonstrating the genetic stability of this accumulation system.

Multiple recent horizontal transfers of the cox1 intron in Solanaceae and extended co-conversion of flanking exons

Background

The most frequent case of horizontal transfer in plants involves a group I intron in the mitochondrial gene cox1, which has been acquired via some 80 separate plant-to-plant transfer events among 833 diverse angiosperms examined. This homing intron encodes an endonuclease thought to promote the intron's promiscuous behavior. A promising experimental approach to study endonuclease activity and intron transmission involves somatic cell hybridization, which in plants leads to mitochondrial fusion and genome recombination. However, the cox1 intron has not yet been found in the ideal group for plant somatic genetics - the Solanaceae. We therefore undertook an extensive survey of this family to find members with the intron and to learn more about the evolutionary history of this exceptionally mobile genetic element.

Results

Although 409 of the 426 species of Solanaceae examined lack the cox1 intron, it is uniformly present in three phylogenetically disjunct clades. Despite strong overall incongruence of cox1 intron phylogeny with angiosperm phylogeny, two of these clades possess nearly identical intron sequences and are monophyletic in intron phylogeny. These two clades, and possibly the third also, contain a co-conversion tract (CCT) downstream of the intron that is extended relative to all previously recognized CCTs in angiosperm cox1. Re-examination of all published cox1 genes uncovered additional cases of extended co-conversion and identified a rare case of putative intron loss, accompanied by full retention of the CCT.

Conclusions

We infer that the cox1 intron was separately and recently acquired by at least three different lineages of Solanaceae. The striking identity of the intron and CCT from two of these lineages suggests that one of these three intron captures may have occurred by a within-family transfer event. This is consistent with previous evidence that horizontal transfer in plants is biased towards phylogenetically local events. The discovery of extended co-conversion suggests that other cox1 conversions may be longer than realized but obscured by the exceptional conservation of plant mitochondrial sequences. Our findings provide further support for the rampant-transfer model of cox1 intron evolution and recommend the Solanaceae as a model system for the experimental analysis of cox1 intron transfer in plants.

A cost-effective ELP-intein coupling system for recombinant protein purification from plant production platform

BACKGROUND

Plant bioreactor offers an efficient and economical system for large-scale production of recombinant proteins. However, high cost and difficulty in scaling-up of downstream purification of the target protein, particularly the common involvement of affinity chromatography and protease in the purification process, has hampered its industrial scale application, therefore a cost-effective and easily scale-up purification method is highly desirable for further development of plant bioreactor.

METHODOLOGY/PRINCIPAL FINDINGS

To tackle this problem, we investigated the ELP-intein coupling system for purification of recombinant proteins expressed in transgenic plants using a plant lectin (PAL) with anti-tumor bioactivity as example target protein and rice seeds as production platform. Results showed that ELP-intein-PAL (EiP) fusion protein formed novel irregular ER-derived protein bodies in endosperm cells by retention of endogenous prolamins. The fusion protein was partially self-cleaved in vivo, but only self-cleaved PAL protein was detected in total seed protein sample and deposited in protein storage vacuoles (PSV). The in vivo uncleaved EiP protein was accumulated up to 2-4.2% of the total seed protein. The target PAL protein could be purified by the ELP-intein system efficiently without using complicated instruments and expensive chemicals, and the yield of pure PAL protein by the current method was up to 1.1 mg/g total seed protein.

CONCLUSION/SIGNIFICANCE

This study successfully demonstrated the purification of an example recombinant protein from rice seeds by the ELP-intein system. The whole purification procedure can be easily scaled up for industrial production, providing the first evidence on applying the ELP-intein coupling system to achieve cost-effective purification of recombinant proteins expressed in plant bioreactors and its possible application in industry.

Recombinant expression of homodimeric 660 kDa human thyroglobulin in soybean seeds: an alternative source of human thyroglobulin

Soybean seeds possess many qualities that make them ideal targets for the production of recombinant proteins. However, one quality often overlooked is their ability to stockpile large amounts of complex storage proteins. Because of this characteristic, we hypothesized that soybean seeds would support recombinant expression of large and complex proteins that are currently difficult or impossible to express using traditional plant and non-plant-based host systems. To test this hypothesis, we transformed soybeans with a synthetic gene encoding human thyroglobulin (hTG)-a 660 kDa homodimeric protein that is widely used in the diagnostic industry for screening and detection of thyroid disease. In the absence of a recombinant system that can produce recombinant hTG, research and diagnostic grade hTG continues to be purified from cadaver and surgically removed thyroid tissue. These less-than-ideal tissue sources lack uniform glycosylation and iodination and therefore introduce variability when purified hTG is used in sensitive ELISA screens. In this study, we report the successful expression of recombinant hTG in soybean seeds. Authenticity of the soy-derived protein was demonstrated using commercial ELISA kits developed specifically for the detection of hTG in patient sera. Western analyses and gel filtration chromatography demonstrated that recombinant hTG and thyroid-purified hTG are biologically similar with respect to size, mass, charge and subunit interaction. The recombinant protein was stable over three generations and accumulated to ~1.5% of total soluble seed protein. These results support our hypothesis that soybeans represent a practical alternative to traditional host systems for the expression of large and complex proteins.

Production of biologically active human thioredoxin 1 protein in lettuce chloroplasts

The production of human therapeutic proteins in plants provides opportunities for low-cost production, and minimizes the risk of contamination from potential human pathogens. Chloroplast genetic engineering is a particularly promising strategy, because plant chloroplasts can produce large amounts of foreign target proteins. Oxidative stress is a key factor in various human diseases. Human thioredoxin 1 (hTrx1) is a stress-induced protein that functions as an antioxidant against oxidative stress, and overexpression of hTrx1 has been shown to suppress various diseases in mice. Therefore, hTrx1 is a prospective candidate as a new human therapeutic protein. We created transplastomic lettuce expressing hTrx1 under the control of the psbA promoter. Transplastomic plants grew normally and were fertile. The hTrx1 protein accumulated to approximately 1% of total soluble protein in mature leaves. The hTrx1 protein purified from lettuce leaves was functionally active, and reduced insulin disulfides. The purified protein protected mouse insulinoma line 6 cells from damage by hydrogen peroxide, as reported previously for a recombinant hTrx1 expressed in Escherichia coli. This is the first report of expression of the biologically active hTrx1 protein in plant chloroplasts. This research opens up possibilities for plant-based production of hTrx1. Considering that this expression host is an edible crop plant, this transplastomic lettuce may be suitable for oral delivery of hTrx1.

Expression of biologically active human insulin-like growth factor 1 in Arabidopsis thaliana seeds via oleosin fusion technology

Novel protein expression in plant-based systems has become an important tool in producing and studying therapeutic proteins. Among many plant-based systems developed so far, oleosin fusion technology is one of the most cost-effective and convenient methods. In this study, an important therapeutic protein, human insulin-like growth factor 1 (hIGF-1), was expressed in Arabidopsis thaliana seeds via this technology. The plant bias codon usage-optimized hIGF-1 gene was fused to the C-terminal of A. thaliana 18.5 kDa oleosin gene, and the fusion gene driven by an oleosin promoter was transferred into A. thaliana ecotype Col-0. The accumulation of oleosin-hIGF-1 fusion protein in transgenic seeds was up to 0.75% of total seed protein (TSP) and the expression level of hIGF-1 was 0.17% of the TSP, which was eight times higher than previously reported using other plant-based hIGF-1 production systems. The biological activity of the hIGF-1 as an oleosin-hIGF-1 fusion protein in vitro was demonstrated by using human SH-SY5Y neuroblastoma cells.

Expression of a staphylokinase, a thrombolytic agent in Arabidopsis thaliana

A gene encoding staphylokinase from Staphylococcus aureus was cloned into the plant transformation binary vector pCAMBIA 1304. The transgene was introduced into the genome of A. thaliana via in planta Agrobacterium tumefaciens-mediated genetic transformation. The presence of the staphylokinase gene was confirmed by PCR in 60% of the investigated plants. The presence of the fusion protein (119 kDa) was confirmed by SDS-PAGE and Western blot analysis in protein extracts from putative transgenics. Furthermore, the amidolytic assay confirmed the activity of SAK in protein extracts in 23 out of 45 transgenic lines of A. thaliana plants.

Evolution of plant-made pharmaceuticals

The science and policy of pharmaceuticals produced and/or delivered by plants has evolved over the past twenty-one years from a backyard remedy to regulated, purified products. After seemingly frozen at Phase I human clinical trials with six orally delivered plant-made vaccines not progressing past this stage over seven years, plant-made pharmaceuticals have made a breakthrough with several purified plant-based products advancing to Phase II trials and beyond. Though fraught with the usual difficulties of pharmaceutical development, pharmaceuticals made by plants have achieved pertinent milestones albeit slowly compared to other pharmaceutical production systems and are now at the cusp of reaching the consumer. Though the current economic climate begs for cautious investment as opposed to trail blazing, it is perhaps a good time to look to the future of plant-made pharmaceutical technology to assist in planning for future developments in order not to slow this technology's momentum. To encourage continued progress, we highlight the advances made so far by this technology, particularly the change in paradigms, comparing developmental timelines, and summarizing the current status and future possibilities of plant-made pharmaceuticals.

Protein body-inducing fusions for high-level production and purification of recombinant proteins in plants

For the past two decades, therapeutic and industrially important proteins have been expressed in plants with varying levels of success. The two major challenges hindering the economical production of plant-made recombinant proteins include inadequate accumulation levels and the lack of efficient purification methods. To address these limitations, several fusion protein strategies have been recently developed to significantly enhance the production yield of plant-made recombinant proteins, while simultaneously assisting in their subsequent purification. Elastin-like polypeptides are thermally responsive biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the purification of recombinant proteins. Hydrophobins are small fungal proteins capable of altering the hydrophobicity of their respective fusion partner, thus enabling efficient purification by surfactant-based aqueous two-phase systems. Zera, a domain of the maize seed storage protein γ-zein, can induce the formation of protein storage bodies, thus facilitating the recovery of fused proteins using density-based separation methods. These three novel protein fusion systems have also been shown to enhance the accumulation of a range of different recombinant proteins, while concurrently inducing the formation of protein bodies. The packing of these fusion proteins into protein bodies may exclude the recombinant protein from normal physiological turnover. Furthermore, these systems allow for quick, simple and inexpensive nonchromatographic purification of the recombinant protein, which can be scaled up to industrial levels of protein production. This review will focus on the similarities and differences of these artificial storage organelles, their biogenesis and their implication for the production of recombinant proteins in plants and their subsequent purification.

Plant bioreactors for pharmaceuticals

Plant bioreactors are attractive expression systems for economic production of pharmaceuticals. Various plant expression systems or platforms have been tested with certain degrees of success over the past years. However, further development and improvement are needed for more effective plant bioreactors. In this review we first summarize recent progress in various plant bioreactor expression systems and then focus on discussing protein compartmentation to unique organelles and various strategies for developing better plant bioreactors.

Deletion of fucose residues in plant N-glycans by repression of the GDP-mannose 4,6-dehydratase gene using virus-induced gene silencing and RNA interference

Production of pharmaceutical glycoproteins in plants has many advantages in terms of safety and reduced costs. However, plant-produced glycoproteins have N-glycans with plant-specific sugar residues (core β-1,2-xylose and α-1,3-fucose) and a Lewis a (Le(a) ) epitope, i.e., Galβ(1-3)[Fucα(1-4)]GlcNAc. Because these sugar residues and glycan structures seemed to be immunogenic, several attempts have been made to delete them by repressing their respective glycosyltransferase genes. However, until date, such deletions have not been successful in completely eliminating the fucose residues. In this study, we simultaneously reduced the plant-specific core α-1,3-fucose and α-1,4-fucose residues in the Le(a) epitopes by repressing the Guanosine 5'-diphosphate (GDP)-D-mannose 4,6-dehydratase (GMD) gene, which is associated with GDP-L-fucose biosynthesis, in Nicotiana benthamiana plants. Repression of GMD was achieved using virus-induced gene silencing (VIGS) and RNA interference (RNAi). The proportion of fucose-free N-glycans found in total soluble protein from GMD gene-repressed plants increased by 80% and 95% following VIGS and RNAi, respectively, compared to wild-type plants. A small amount of putative galactose substitution in N-glycans from the NbGMD gene-repressed plants was observed, similar to what has been previously reported GMD-knockout Arabidopsis mutant. On the other hand, the recombinant mouse granulocyte-macrophage colony-stimulating factor (GM-CSF) with fucose-deleted N-glycans was successfully produced in NbGMD-RNAi transgenic N. benthamiana plants. Thus, repression of the GMD gene is thus very useful for deleting immunogenic total fucose residues and facilitating the production of pharmaceutical glycoproteins in plants.

High-level accumulation of recombinant miraculin protein in transgenic tomatoes expressing a synthetic miraculin gene with optimized codon usage terminated by the native miraculin terminator

In our previous study, a transgenic tomato line that expressed the MIR gene under control of the cauliflower mosaic virus 35S promoter and the nopaline synthase terminator (tNOS) produced the taste-modifying protein miraculin (MIR). However, the concentration of MIR in the tomatoes was lower than that in the MIR gene's native miracle fruit. To increase MIR production, the native MIR terminator (tMIR) was used and a synthetic gene encoding MIR protein (sMIR) was designed to optimize its codon usage for tomato. Four different combinations of these genes and terminators (MIR-tNOS, MIR-tMIR, sMIR-tNOS and sMIR-tMIR) were constructed and used for transformation. The average MIR concentrations in MIR-tNOS, MIR-tMIR, sMIR-tNOS and sMIR-tMIR fruits were 131, 197, 128 and 287 μg/g fresh weight, respectively. The MIR concentrations using tMIR were higher than those using tNOS. The highest MIR accumulation was detected in sMIR-tMIR fruits. On the other hand, the MIR concentration was largely unaffected by sMIR-tNOS. The expression levels of both MIR and sMIR mRNAs terminated by tMIR tended to be higher than those terminated by tNOS. Read-through mRNA transcripts terminated by tNOS were much longer than those terminated by tMIR. These results suggest that tMIR enhances mRNA expression and permits the multiplier effect of optimized codon usage.