Molecular farming in plants: host systems and expression technology

Barley as a green factory for the production of functional Flt3 ligand

Biologically active recombinant human Flt3 ligand was expressed and isolated from transgenic barley seeds. Its expression is controlled by a tissue specific promoter that confines accumulation of the recombinant protein to the endosperm tissue of the seed. The recombinant Flt3 ligand variant expressed in the seeds contains an HQ-tag for affinity purification on immobilized metal ion affinity chromatography (IMAC) resin. The tagged protein was purified from seed extracts to near homogeneity using sequential chromatography on IMAC affinity resin and cation exchange resin. We also show that the recombinant Flt3 ligand protein undergoes posttranslational modifications: it is a glycoprotein containing alpha-1,3-fucose and alpha-1,2-xylose. The HQ-tagged Flt3 ligand variant exhibits comparable biological activity to commercial Flt3 ligand. This is the first report showing expression and accumulation of recombinant human growth factor in barley seeds with a yield of active protein similar to a bacterial expression system. The present results demonstrate that plant molecular farming is a viable approach for the bioproduction of human-derived growth factors.

Temporal and spatial distribution of erythropoietin in transgenic tobacco plants

Plants have shown promise as bioreactors for the large-scale production of a wide variety of recombinant proteins. To increase the economic feasibility of this technology, numerous molecular approaches have been developed to enhance the production yield of these valuable proteins in plants. Alternatively, we chose to examine the temporal and spatial distribution of erythropoietin (EPO) accumulation during tobacco plant development, in order to establish the optimal harvesting time to further maximize heterologous protein recovery. EPO is used extensively worldwide for the treatment of anaemia and is currently the most commercially valuable biopharmaceutical on the market. Our results indicate that the concentration of recombinant EPO and endogenous total soluble protein (TSP) declined significantly for every leaf of the plant during maturation, although the rate of these declines was strongly dependent on the leaf's position on the plant. As a result, the amount of EPO produced in leaves relative to TSP content remained essentially unchanged over the course of the plant's life. Decreasing levels of recombinant protein in leaves was attributed to proteolytic degradation associated with tissue senescence since transgene silencing was not detected. We found that significantly higher concentrations of EPO within younger leaves more than compensated for their smaller size, when compared to their low-expressing, fully-grown counterparts. This suggests that fast-growing, young leaves should be periodically harvested from the plants as they continue to grow in order to maximize recombinant protein yield. These findings demonstrate that EPO accumulation is highly influenced by the plant's physiology and development.

Recombinant mouse granulocyte-macrophage colony-stimulating factor is glycosylated in transgenic tobacco and maintains its biological activity

The granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine with many important applications and, due to its immunostimulatory properties, could also be used as a vaccine adjuvant. A simple strategy to produce recombinant mouse GM-CSF (mGM-CSF) in transgenic Nicotiana tabacum plants was used in this study. The mGM-CSF cDNA followed by the sequence encoding endoplasmic reticulum retention signal (KDEL) was cloned into the ImpactVector under the control of the strong promoter from the gene encoding a small subunit of Rubisco. In transgenic plants the accumulation level of recombinant mGM-CSF varied in the individual transformants from 8 to 19 microg/g of fresh leaf tissue, which makes up to 0.22% of total soluble protein. In most analyzed plants, the apparent molecular weight of the recombinant protein was larger than predicted due to its N-glycosylation, presumably in 2 sites. The recombinant plant-produced murine GM-CSF retained its biological activity as confirmed in vitro in proliferation assay using a mouse cell line, which is growth-dependent on GM-CSF.

Production of heterologous proteins in plants: strategies for optimal expression

Plants are a promising expression system for the production of heterologous proteins, especially therapeutic proteins. Currently the majority of therapeutic proteins are produced in mammalian cell lines or bacteria. In a few cases insects, yeast and fungi have been developed for production of human proteins. However, these expression systems have limitations in terms of suitability, cost, scalability, purification and post-translational modifications. Therefore, alternative expression systems are being developed in transgenic animals and transgenic plants. Transgenic plants could provide an attractive alternative in terms of low production cost and lower capital investment in infrastructure, and with appropriate post-translational modifications. The potential of plants as an expression host has not been capitalized, primarily due to lower level of expression of transgenes in plants. The present review will evaluate the rate limiting steps of plant expression systems and suggest strategies to optimize protein expression at each of the steps: gene integration, transcription, translation and protein accumulation.

Genetically modified plants for non-food or non-feed purposes: straightforward screening for their appearance in food and feed

Genetically modified (GM) plants aimed at producing food/feed are part of regular agriculture in many areas of the World. Commodity plants have also found application as bioreactors, designated non-food/non-feed GM (NFGM) plants, thereby making raw material for further refinement to industrial, diagnostic or pharmaceutical preparations. Many among them may pose health challenge to consumers or livestock animals, if occurring in food/feed. NFGM plants are typically released into the environment, but are grown under special oversight and any among several containment practices, none of which provide full protection against accidental dispersal. Adventitious admixture with food or feed can occur either through distributional mismanagement or as a consequence of gene flow to plant relatives. To facilitate NFGM surveillance we propose a new mandatory tagging of essentially all such plants, prior to cultivation or marketing in the European Union. The suggested tag--Plant-Made Industrial or Pharmaceutical Products Tag (PMIP-T)--is envisaged to occur as a transgenic silent DNA identifier in host plants and designed to enable technically simple identification and characterisation of any NFGM. Implementation of PMIP-T would permit inexpensive, reliable and high-throughput screening for NFGM specifically. The paper outlines key NFGM prospects and challenges as well as the PMIP-T concept.

A leaf-based assay using interchangeable design principles to rapidly assemble multistep recombinant pathways

The assembly of multistep recombinant pathways in stably transformed plants is a cornerstone of crops producing new products yet can be a laborious and time-consuming process. Any heterologous expression platform capable of providing a rapid estimation of the functional assembly of an entire pathway would guide the design of such transgenic traits. In this study, we use a Nicotiana benthamiana transient leaf expression system to simultaneously express five genes, from five independent T(DNA) binary vectors, to assemble a complete recombinant pathway in five days. In this study, we demonstrate the production of long-chain polyunsaturated fatty acids (LC-PUFA) requiring five transgene-encoded reactions to convert endogenous fatty acids to LC-PUFA. The addition of a triacylglycerol assembly enzyme, Arabidopsis thaliana diacylglyceride-O-acyltransferase, and fractionation of the total lipid profile demonstrated that leaf oils contained 37% newly synthesised LC-PUFA, including 7% arachidonic acid (AA), 6% eicosopentaenoic acid and 3% docosahexaenoic acid. The calculation of enzymatic conversion efficiencies at each step of LC-PUFA synthesis suggests that this transient assembly of a complicated multistep pathway is highly efficient. Unlike experiments using stably transformed plants our assembly of an intricate pathway maintained full gene-for-gene interchangeability and required a fraction of the time and glasshouse space. Furthermore, an exogenous LC-PUFA fatty acid substrate, AA, was fed and metabolised by a transiently expressed Delta17-desaturase enzyme, and provided results similar to those obtained in yeast feeding experiments. Although the assay was ideal for LC-PUFA pathways, this assay format may become a powerful tool for the characterisation and step-wise improvement of other recombinant pathways and multigenic traits.

Dunaliella as an attractive candidate for molecular farming

Pharmaceutical recombinant proteins are widely used in human healthcare. At present, several protein expression systems are available to generate therapeutic proteins. These conventional systems have distinct advantages and disadvantages in protein yielding; in terms of ease of manipulation, the time required from gene transformation to protein purification, cost of production and scaling-up capitalization, proper folding and stability of active proteins. Depending on the research goal and priorities, a special system may be selected for protein expression. However, considering the limited variety of organisms currently used and their usage restrictions, there are still much more pharmaceutical proteins waiting to be economically and efficiently produced. Distinguished biological and technical features of microalgae Dunaliella such as inexpensive medium requirement, fast growth rate, the ease of manipulation, easy scaling up procedure, facility of milking in bioreactors and the ability of post-translational modifications make this microorganism an attractive candidate for molecular farming.

Combinatorial de novo design and application of a biomimetic affinity ligand for the purification of human anti-HIV mAb 4E10 from transgenic tobacco

Monoclonal anti-HIV antibody 4E10 (mAb 4E10) is one of the most broadly neutralizing antibodies against HIV, directed against a specific epitope on envelope protein gp41. In the present study, a combinatorial de novo design approach was used for the development of a biomimetic ligand for the affinity purification of mAb 4E10 from tobacco transgenic extract in a single chromatographic step. The biomimetic ligand (4E10lig) was based on a L-Phe/beta-Ala bi-substituted 1,3,5-triazine (Trz) scaffold (beta-Ala-Trz-L-Phe, 4E10lig) which potentially mimics the more pronounced electrostatic and hydrophobic interactions of mAb 4E10-binding sequence determined by screening of a random peptide library. This library was comprised of Escherichia coli cells harboring a plasmid (pFlitrx) engineered to express a fusion protein containing random dodecapeptides that were inserted into the active loop of thioredoxin, which itself was inserted into the dispensable region of the flagellin gene. Adsorption equilibrium studies with this biomimetic ligand and mAb 4E10 determined a dissociation constant (K(D)) of 0.41 +/- 0.05 microM. Molecular modeling studies of the biomimetic ligand revealed that it can potentially occupy the same binding site as the natural binding core peptide epitope. The biomimetic affinity adsorbent was exploited in the development of a facile mAb 4E10 purification protocol, affording mAb 4E10 of high purity (approximately 95%) with good overall yield (60-80%). Analysis of the antibody preparation by SDS-PAGE, enzyme-linked immunosorbent assays (ELISA), and western blot showed that the mAb 4E10 was fully active and free of degraded variants, polyphenols, and alkaloids.

Expression and purification of an anti-Foot-and-mouth disease virus single chain variable antibody fragment in tobacco plants

Low-cost recombinant antibodies could provide a new strategy to control Foot-and-mouth disease virus (FMDV) outbreaks by passive immunization of susceptible animals. In this study, a single chain variable antibody fragment (scFv) recognizing FMDV coat protein VP1 was expressed in transgenic tobacco plants. To enhance the accumulation of scFv protein, the codon-usage of a murine hybridoma-derived scFv gene was adjusted to mimic highly expressed tobacco genes and fused to an elastin-like polypeptide (ELP) tag. This scFv-ELP fusion accumulated up to 0.8% of total soluble leaf protein in transgenic tobacco. To recover scFv-ELP protein from the leaf extract, a simple and scalable purification strategy was established. Purified scFv-ELP fusion was cleaved to separate the scFv portion. Finally, it was shown that the purified scFv proteins retained their capacity to bind the FMDV in the absence or presence of ELP fusion.

Production of recombinant single chain antibodies (scFv) in vegetatively reproductive Kalanchoe pinnata by in planta transformation

We developed an asexual reproductive plant, Kalanchoe pinnata, as a new bioreactor for plant-based molecular farming using a newly developed transformation method. Leaf crenate margins were pin-pricked to infect the plant with the Agrobacterium strain LBA4404 and vacuum infiltration was also applied to introduce the target gene into the plants. Subsequently, the young mother leaf produced new clones at the leaf crenate margins without the need for time- and labor-consuming tissue culture procedures. The average transformation rates were approximately 77 and 84% for pin-prickling and vacuum-infiltration methods, respectively. To functionally characterize an introduced target protein, a nucleic acid hydrolyzing recombinant 3D8 scFv was selected and the plant based 3D8 scFv proteins were purified and analyzed. Based on abzyme analysis, the purified protein expressed with this system had catalytic activity and exhibited all of properties of the protein produced in an E. coli system. This result suggested that vegetatively reproductive K. pinnata can be a novel and potent bioreactor for bio-pharmaceutical proteins.

Acquired resistance to the rice blast in transgenic rice accumulating the antimicrobial peptide thanatin

Thanatin is an antimicrobial peptide with a strong and wide-ranging antimicrobial spectrum, including certain species of fungi and Gram-negative and Gram-positive bacteria. To evaluate the application of thanatin to the generation of disease-resistant plants, we introduced a synthetic thanatin gene into rice. Several transformants that expressed the introduced gene showed significant level of antimicrobial activity. The substances showing antimicrobial activity were partially purified from these transformants and their properties were determined. The molecule with characteristics similar to those of native thanatin on the elution pattern in HPLC analysis had an identical molecular mass to that of native molecule. It should also be noted that the transformant acquired a sufficient level of resistance to the rice blast fungus, Magnaporthe oryzae, presumably due to the repressive activity of thanatin to its initial stage of infection. This result demonstrates that thanatin has antifungal activity for M. oryzae and that the introduction of the thanatin gene into rice is effective in generating a plant resistant to rice blast disease.

Production of bioactive, post-translationally modified, heterotrimeric, human recombinant type-I collagen in transgenic tobacco

Collagen's biocompatibility, biodegradability and low immunogenicity render it advantageous for extensive application in pharmaceutical or biotechnological disciplines. However, typical collagen extraction from animal or cadaver sources harbors risks including allergenicity and potential sample contamination with pathogens. In this work, two human genes encoding recombinant heterotrimeric collagen type I (rhCOL1) were successfully coexpressed in tobacco plants with the human prolyl-4-hydroxylase (P4H) and lysyl hydroxylase 3 (LH3) enzymes, responsible for key posttranslational modifications of collagen. Plants coexpressing all five vacuole-targeted proteins generated intact procollagen yields of approximately 2% of the extracted total soluble proteins. Plant-extracted rhCOL1 formed thermally stable triple helical structures and demonstrated biofunctionality similar to human tissue-derived collagen supporting binding and proliferation of adult peripheral blood-derived endothelial progenitor-like cells. Through a simple, safe and scalable method of rhCOL1 production and purification from tobacco plants, this work broadens the potential applications of human recombinant collagen in regenerative medicine.

In vitro propagation of plant virus using different forms of plant tissue culture and modes of culture operation

Plant virus accumulation was investigated in vitro using three different forms of plant tissue culture. Suspended cells, hairy roots and shooty teratomas of Nicotiana benthamiana were infected with tobacco mosaic virus (TMV) using the same initial virus:biomass ratio. Viral infection did not affect tissue growth or morphology in any of the three culture systems. Average maximum virus concentrations in hairy roots and shooty teratomas were similar and about an order of magnitude higher than in suspended cells. Hairy roots were considered the preferred host because of their morphological stability in liquid medium and relative ease of culture. The average maximum virus concentration in the hairy roots was 0.82+/-0.14 mg g(-1) dry weight; viral coat protein represented a maximum of approximately 6% of total soluble protein in the biomass. Virus accumulation in hairy roots was investigated further using different modes of semi-continuous culture operation aimed at prolonging the root growth phase and providing nutrient supplementation; however, virus concentrations in the roots were not enhanced compared with simple batch culture. The relative infectivity of virus in the biomass declined by 80-90% during all the cultures tested, irrespective of the form of plant tissue used or mode of culture operation. Hairy root cultures inoculated with a transgenic TMV-based vector in batch culture accumulated green fluorescent protein (GFP); however, maximum GFP concentrations in the biomass were relatively low at 39 microg g(-1) dry weight, probably due to genetic instability of the vector. This work highlights the advantages of using hairy roots for in vitro propagation of TMV compared with shooty teratomas and suspended plant cells, and demonstrates that batch root culture is more effective than semi-continuous operations for accumulation of high virus concentrations in the biomass.

Role of genetic factors and environmental conditions in recombinant protein production for molecular farming

Plants are generally considered to represent a promising heterologous expression system for the production of valuable recombinant proteins. Minimal upstream plant production cost is a salient feature driving the development of plant expression systems used for the synthesis of recombinant proteins. For such a plant expression system to be fully effective, it is first essential to improve plant productivity by plant biomass after inserting genes of interest into a suitable plant. Plant productivity is related closely to its growth and development, both of which are affected directly by environmental factors. These environmental factors that affect the cultivation conditions mainly include temperature, light, salinity, drought, nutrition, insects and pests. In addition, genetic factors that affect gene expression at the transcriptional, translational, and post-translational levels are considered to be important factors related to gene expression in plants. Thus, these factors influence both the quality and quantity of recombinant protein produced in transgenic plants. Among the genetic factors, the post-translational process is of particular interest as it influences subcellular localization, protein glycosylation, assembly and folding of therapeutic proteins, consequently affecting both protein quantity and biological quality. In this review, we discuss the effects of cultivation condition and genetic factors on recombinant protein production in transgenic plants.

Induction of protein body formation in plant leaves by elastin-like polypeptide fusions

BACKGROUND

Elastin-like polypeptides are synthetic biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the simple non-chromatographic purification of recombinant proteins. In addition, elastin-like polypeptide fusions have been shown to enhance the accumulation of a range of different recombinant proteins in plants, thus addressing the major limitation of plant-based expression systems, which is a low production yield. This study's main objectives were to determine the general utility of elastin-like polypeptide protein fusions in various intracellular compartments and to elucidate elastin-like polypeptide's mechanism of action for increasing recombinant protein accumulation in the endoplasmic reticulum of plants.

RESULTS

The effect of elastin-like polypeptide fusions on the accumulation of green fluorescent protein targeted to the cytoplasm, chloroplasts, apoplast, and endoplasmic reticulum was evaluated. The endoplasmic reticulum was the only intracellular compartment in which an elastin-like polypeptide tag was shown to significantly enhance recombinant protein accumulation. Interestingly, endoplasmic reticulum-targeted elastin-like polypeptide fusions induced the formation of a novel type of protein body, which may be responsible for elastin-like polypeptide's positive effect on recombinant protein accumulation by excluding the heterologous protein from normal physiological turnover. Although expressed in the leaves of plants, these novel protein bodies appeared similar in size and morphology to the prolamin-based protein bodies naturally found in plant seeds. The elastin-like polypeptide-induced protein bodies were highly mobile organelles, exhibiting various dynamic patterns of movement throughout the cells, which were dependent on intact actin microfilaments and a functional actomyosin motility system.

CONCLUSION

An endoplasmic reticulum-targeted elastin-like polypeptide fusion approach provides an effective strategy for depositing large amounts of concentrated heterologous protein within the limited space of the cell via storage in stable protein bodies. Furthermore, encapsulation of recombinant proteins into physiologically inert organelles can function to insulate the protein from normal cellular mechanisms, thus limiting unnecessary stress to the host cell. Since elastin-like polypeptide is a mammalian-derived protein, this study demonstrates that plant seed-specific factors are not required for the formation of protein bodies in vegetative plant tissues, suggesting that the endoplasmic reticulum possesses an intrinsic ability to form protein body-like accretions in eukaryotic cells when overexpressing particular proteins.

Expression of Acidothermus cellulolyticus E1 endo-beta-1,4-glucanase catalytic domain in transplastomic tobacco

As part of an effort to develop transgenic plants as a system for the production of lignocellulose-degrading enzymes, we evaluated the production of the endo-beta-1,4-glucanase E1 catalytic domain (E1cd) of Acidothermus cellulolyticus in transplastomic tobacco. In an attempt to increase the translation efficiency of the E1cd cassette, various lengths of the N-terminus of the psbA gene product were fused to the E1cd protein. The psbA gene of the plastid genome encodes the D1 polypeptide of photosystem II and is known to encode an efficiently translated mRNA. Experiments in an Escherichia coli expression system indicated that the fusion of short (10-22 amino acid) segments of D1 to E1cd resulted in modest increases in E1cd abundance and were compatible with E1cd activity. Plastid expression cassettes encoding unmodified E1cd and a 10-amino-acid D1 fusion (10nE1cd) were used to generate transplastomic tobacco plants. Expression of the E1cd open reading frame in transplastomic tobacco resulted in very low levels of the enzyme. The transplastomic plants accumulated a high level of E1cd mRNA, however, indicating that post-transcriptional processes were probably limiting the production of recombinant protein. The accumulation of 10nE1cd in transplastomic tobacco was approximately 200-fold higher than that of unmodified E1cd, yielding 10nE1cd in excess of 12% of total soluble protein in the extracts of the lower leaves. Most importantly, the active recombinant enzyme was recovered very easily and efficiently from dried plant material and constituted as much as 0.3% of the dry weight of leaf tissue.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.

Optimization of elastin-like polypeptide fusions for expression and purification of recombinant proteins in plants

The demand for recombinant proteins for medical and industrial use is expanding rapidly and plants are now recognized as an efficient, inexpensive means of production. Although the accumulation of recombinant proteins in transgenic plants can be low, we have previously demonstrated that fusions with an elastin-like polypeptide (ELP) tag can significantly enhance the production yield of a range of different recombinant proteins in plant leaves. ELPs are biopolymers with a repeating pentapeptide sequence (VGVPG)(n) that are valuable for bioseparation, acting as thermally responsive tags for the non-chromatographic purification of recombinant proteins. To determine the optimal ELP size for the accumulation of recombinant proteins and their subsequent purification, various ELP tags were fused to green fluorescent protein, interleukin-10, erythropoietin and a single chain antibody fragment and then transiently expressed in tobacco leaves. Our results indicated that ELP tags with 30 pentapeptide repeats provided the best compromise between the positive effects of small ELP tags (n = 5-40) on recombinant protein accumulation and the beneficial effects of larger ELP tags (n = 80-160) on recombinant protein recovery during inverse transition cycling (ITC) purification. In addition, the C-terminal orientation of ELP fusion tags produced higher levels of target proteins, relative to N-terminal ELP fusions. Importantly, the ELP tags had no adverse effect on the receptor binding affinity of erythropoietin, demonstrating the inert nature of these tags. The use of ELP fusion tags provides an approach for enhancing the production of recombinant proteins in plants, while simultaneously assisting in their purification.

Foreign protein production using plant cell and organ cultures: Advantages and limitations

Plants and plant tissue cultures are used as host systems for expression of foreign proteins including antibodies, vaccines and other therapeutic agents. Recombinant or stably transformed plants and plant cell cultures have been applied for foreign protein production for about 20 years. Because the product concentration achieved exerts a major influence on process economics, considerable efforts have been made by commercial and academic research groups to improve foreign protein expression levels. However, post-synthesis product losses due to protease activity within plant tissues and/or extracellular protein adsorption in plant cell cultures can negate the benefits of molecular or genetic enhancement of protein expression. Transient expression of foreign proteins using plant viral vectors is also a practical approach for producing foreign proteins in plants. Adaptation of this technology is required to allow infection and propagation of engineered viruses in plant tissue cultures for transient protein expression in vitro.

Optimization of the bioprocessing conditions for scale-up of transient production of a heterologous protein in plants using a chemically inducible viral amplicon expression system

Use of transient expression for the rapid, large-scale production of recombinant proteins in plants requires optimization of existing methods to facilitate scale-up of the process. We have demonstrated that the techniques used for agroinfiltration and induction greatly impact transient production levels of heterologous protein. A Cucumber mosaic virus inducible viral amplicon (CMViva) expression system was used to transiently produce recombinant alpha-1-antitrypsin (rAAT) by co-infiltrating harvested Nicotiana benthamiana leaves with two Agrobacterium tumefaciens strains, one containing the CMViva expression cassette carrying the AAT gene and the other containing a binary vector carrying the gene silencing suppressor p19. Harvested leaves were both infiltrated and induced by either pressure or vacuum infiltration. Using the vacuum technique for both processes, maximum levels of functional and total rAAT were elevated by (190 +/- 8.7)% and (290 +/- 7.5)%, respectively, over levels achieved when using the pressure technique for both processes. The bioprocessing conditions for vacuum infiltration and induction were optimized and resulted in maximum rAAT production when using an A. tumefaciens concentration at OD(600) of 0.5 and a 0.25-min vacuum infiltration, and multiple 1-min vacuum inductions further increased production 25% and resulted in maximum levels of functional and total rAAT at (2.6 +/- 0.09)% and (4.1 +/- 0.29)% of the total soluble protein, respectively, or (90 +/- 1.7) and (140 +/- 10) mg per kg fresh weight leaf tissue at 6 days post-induction. Use of harvested plant tissue with vacuum infiltration and induction demonstrates a bioprocessing route that is fully amenable to scale-up.

Antibody production by the gram-positive bacterium Bacillus megaterium

The increasing demand for recombinant antibodies as detection reagents in research, diagnostics, and therapy requires appropriate production systems. In contrast to antibody therapies, small recombinant antibody fragments like Fab and scFv are sufficient for most applications in research and diagnostics. These antibody fragments can also be produced in bacterial hosts. Gram-negative bacteria, particularly Escherichia coli, were extensively studied for the recombinant antibody production but they showed only a limited capacity to secrete antibody fragments into the medium--a prerequisite for easy downstream processing. Gram-positive bacteria are known to efficiently secrete recombinant proteins into the medium. Recently, we demonstrated the production of scFv and scFab fragments in Bacillus megaterium. Here, we describe the process in detail from transformation of B. megaterium to production and purification of scFv fragments.

Deposition of a recombinant peptide in ER-derived protein bodies by retention with cysteine-rich prolamins in transgenic rice seed

A 7Crp peptide composed of seven major human T cell epitopes derived from the Japanese cedar pollen allergens Cry j 1 and Cry j 2 is an ideal tolerogen for peptide immunotherapy against Japanese cedar pollinosis. To maximize the accumulation level of the 7Crp peptide in transgenic rice seed, we tested endosperm specific promoters and intracellular localizations suitable for stable accumulation. A 7Crp peptide carrying the KDEL ER retention signal directed by the 2.3-kb promoter of the glutelin GluB-1, which contains a signal peptide, accumulated at the highest level of about 60 microg/grain. Notably, the 7Crp peptide predominantly accumulated in ER-derived protein bodies irrespective of the presence of various sorting signals or expression as a fusion protein with glutelin. We attribute this abnormal pattern of accumulation to the formation of disulfide bonds between the 7Crp peptide and cysteine-rich (Cys-rich) prolamin storage proteins. Furthermore, the formation of these aggregates induced the chaperone proteins BiP and PDI as an ER stress response.

High-level expression of the HIV-1 Pr55gag polyprotein in transgenic tobacco chloroplasts

Plants have been recognized as a promising production platform for recombinant pharmaceutical proteins. The human immunodeficiency virus Gag (Pr55(gag)) structural polyprotein precursor is a prime candidate for developing a HIV-1 vaccine, but, so far, has been expressed at very low level in plants. The aim of this study was to investigate factors potentially involved in Pr55(gag) expression and increase protein yield in plant cells. In transient expression experiments in various subcellular compartments, the native Pr55(gag) sequence could be expressed only in the chloroplast. Experiments with truncated subunits suggested a negative role of the 5'-end on the expression of the full gene in the cytosol. Stable transgenic plants were produced in tobacco by Agrobacterium-mediated nuclear transformation with protein targeted to plastids, and biolistic-mediated plastid transformation. Compared to the nuclear genome, the integration and expression of the gag transgene in the plastome resulted in significantly higher protein accumulation levels (up to 7-8% TSP, equivalent to 312-363 mg/kg FW). In transplastomic plants, a 25-fold higher protein accumulation was obtained by translationally fusing the Pr55(gag) polyprotein to the N-terminus of the plastid photosynthetic RbcL protein. In chloroplasts, the Pr55(gag) polyprotein was processed in a pattern similar to that achieved by the viral protease, the processing being more extended in older leaves of mature plants. The Gag proteins produced in transgenic plastids were able to assemble into particles resembling VLPs produced in baculovirus/insect cells and E. coli systems. These results indicate that plastid transformation is a promising tool for HIV antigen manufacturing in plant cells.

Viral and murine interleukin-10 are correctly processed and retain their biological activity when produced in tobacco

BACKGROUND

Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine, with therapeutic applications in several autoimmune and inflammatory diseases. Oral administration of this cytokine alone, or in combination with disease-associated autoantigens could confer protection form the onset of a specific autoimmune disease through the induction of oral tolerance. Transgenic plants are attractive systems for production of therapeutic proteins because of the ability to do large scale-up at low cost, and the low maintenance requirements. They are highly amenable to oral administration and could become effective delivery systems without extensive protein purification. We investigated the ability of tobacco plants to produce high levels of biologically-active viral and murine IL-10.

RESULTS

Three different subcellular targeting strategies were assessed in transient expression experiments, and stable transgenic tobacco plants were generated with the constructs that yielded the highest accumulation levels by targeting the recombinant proteins to the endoplasmic reticulum. The best yields using this strategy in T1 plants were 10.8 and 37.0 microg/g fresh leaf weight for viral and murine IL-10, respectively. The recombinant proteins were purified from transgenic leaf material and characterized in terms of their N-glycan composition, dimerization and biological activity in in vitro assays. Both molecules formed stable dimers, were able to activate the IL-10 signaling pathway and to induce specific anti-inflammatory responses in mouse J774 macrophage cells.

CONCLUSION

Tobacco plants are able to correctly process viral and murine IL-10 into biologically active dimers, therefore representing a suitable platform for the production for these cytokines. The accumulation levels obtained are high enough to allow delivery of an immunologically relevant dose of IL-10 in a reasonable amount of leaf material, without extensive purification. This study paves the way to performing feeding studies in mouse models of autoimmune diseases, that will allow the evaluation the immunomodulatory properties and effectiveness of the viral IL-10 in inducing oral tolerance compared to the murine protein.

High-level bacterial cellulase accumulation in chloroplast-transformed tobacco mediated by downstream box fusions

The Thermobifida fusca cel6A gene encoding an endoglucanase was fused to three different downstream box (DB) regions to generate cel6A genes with 14 amino acid fusions. The DB-Cel6A fusions were inserted into the tobacco (Nicotiana tabacum cv. Samsun) chloroplast genome for protein expression. Accumulation of Cel6A protein in transformed tobacco leaves varied over approximately two orders of magnitude, dependent on the identity of the DB region fused to the cel6A open reading frame (ORF). Additionally, the DB region fused to the cel6A ORF affected the accumulation of Cel6A protein in aging leaves, with the most effective DB regions allowing for high level accumulation of Cel6A protein in young, mature, and old leaves, while Cel6A protein accumulation decreased with leaf age when less effective DB regions were fused to the cel6A ORF. In the most highly expressed DB-Cel6A construct, enzymatically active Cel6A protein accumulated at up to 10.7% of total soluble leaf protein (%TSP). The strategy used for high-level endoglucanase expression may be useful for expression of other cellulolytic enzymes in chloroplasts, ultimately leading to cost-effective heterologous enzyme production for cellulosic ethanol using transplastomic plants.

From Neanderthal to nanobiotech: from plant potions to pharming with plant factories

Plants were the main source for human drugs until the beginning of the nineteenth century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. During the last decades of the twentieth century, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. After a temporary decrease in interest, plants are rapidly moving back into human pharmacopoeia, with the recent development of plant-based recombinant protein production systems offering a safe and extremely cost-effective alternative to microbial and mammalian cell cultures. In this short review, we will illustrate that current improvements in plant expression systems are making them suitable as alternative factories for the production of either simple or highly complex therapeutic proteins.

Production of antibody fragments in Arabidopsis seeds

Plants offer a number of attractive benefits over conventional mammalian or bacterial cell culture systems for the production of valuable pharmaceutical and industrial proteins. Currently, antibodies and their derived fragments represent the largest and most important group of biotechnological products in clinical trials. In particular, single-chain antibodies are an interesting class of biopharmaceuticals because they are able to overcome specific problems associated with full-length antibodies. Another valuable antibody format is the scFv-Fc: fusion of the Fc domain to a single-chain variable fragment restores antibody effector functions, allows purification, and mimics, despite being a 'single-gene' product, the bivalent properties of a full-length IgG. Although many different plant-based production platforms have been evaluated for antibody production, seeds are especially attractive because, as natural storage organs, they provide an optimal biochemical environment for the accumulation and long-term storage of large amounts of functional proteins. This chapter describes how to achieve high-level seed-specific expression of antibody fragments, how to select the best transgenic lines, and how to evaluate the accumulation level in the seed stocks from the selected lines.

Protein body induction: a new tool to produce and recover recombinant proteins in plants

Stable accumulation of storage proteins, lipids and carbohydrates is a hallmark of the plant seed, and is a characteristic that is typically deficient in existing platforms for recombinant protein manufacture. One of the biological sequestration mechanisms that facilitate the folding, assembly and stabilization of plant seed storage proteins involve the de novo formation of unique intracellular organelles, the endoplasmic reticulum (ER)-derived protein bodies (PBs). In cereals, such as maize, PBs are formed directly in the lumen of the ER of endosperm cells and contain zeins, a group of polypeptides, which account for more than half of the total seed protein mass. The 27 kD gamma zein protein localizes to the periphery of the PBs surrounding aggregates of other zeins (including a zein and delta zein). Heterologous expression of gamma zein has been shown to result in the formation of PB-like structures, and the N-terminal proline-rich domain of gamma zein (Zera), containing eight PPPVHL repeats and a Pro-X sequence is by itself capable of directing ER retention and PB formation in non-seed tissues. We present a novel approach to produce recombinant proteins in plants based on the ability of gamma zein-Zera domain to store recombinant proteins inside PBs. Zera domain fused to several proteins, including a enhanced cyan fluorescent protein (ECFP), calcitonin (Ct) and epidermal growth factor (EGF), were cloned into vectors for transient or stable transformation of tobacco plants. In tobacco leaves, we observed the formation of dense, ER-localized structures containing high concentrations of the respective target proteins. The intact synthetic organelles containing Zera fusions were readily isolated from cellular material using density-based separation methods.

Companion protease inhibitors to protect recombinant proteins in transgenic plant extracts

We describe a general approach for the use of recombinant protease inhibitors as stabilizing agents for clinically useful proteins extracted from transgenic plant tissues. A procedure is first described to assess the overall (in)stability of heterologous proteins in transgenic plant crude protein extracts. Step-by-step protocols are then presented for the choice and use of companion protease inhibitors inhibiting the host plant proteases during extraction. This strategy, that reproduces the protein-stabilizing effect of low-molecular-weight protease inhibitors often added to protein extraction media, does not require the exogenous addition of such expensive and often toxic compounds. It also presents the advantage of being intrinsically scalable to the amount of biomass processed.

Biosafety, risk assessment and regulation of plant-made pharmaceuticals

The technology for plant-made pharmaceuticals (PMPs) has progressed significantly over the last few years, with the first commercial products for human use expected to reach the market by 2009 (see Note 1). As part of the 'next generation' of genetically modified (GM) crops, PMPs will be subject to additional biosafety considerations and are set to challenge the complex and overlapping regulations that currently govern GM plants, plant biologics (see Note 2) and 'conventional' pharmaceutical production. The areas of responsibility are being mapped out between the different regulatory agencies (Sparrow, P.A.C., Irwin, J., Dale, P., Twyman, R.M., and Ma, J.K.C. (2007) Pharma-Planta: Road testing the developing regulatory guidelines for plant-made pharmaceuticals. Transgenic Res., 2007), with specific guidelines currently being drawn up for the regulation of PMPs. In this chapter, we provide an overview of the biosafety (see Note 3), risk assessment (see Note 4) and regulation of this emerging technology. While reference will be made to EU regulations, the underlying principles of biosafety and risk assessment are generic to most countries.

New downstream processing strategy for the purification of monoclonal antibodies from transgenic tobacco plants

Affinity chromatography on immobilized Protein A is the current method of choice for the purification of monoclonal antibodies (mAbs). Despite its widespread use it presents certain drawbacks, such as ligand instability, leaching, toxicity and high cost. In the present work, we report a new procedure for the purification of two human monoclonal anti-HIV (human immunodeficiency virus) antibodies (mAbs 2G12 and 4E10) from transgenic tobacco plants using stable and low cost chromatographic materials. The first step of the mAb 2G12 purification procedure is comprised of an aqueous two-phase partition system (ATPS) for the removal of polyphenols while providing an essential initial purification boost (2.01-fold purification). In the second step, mAb 2G12 was purified using cation-exchange chromatography (CEX) on S-Sepharose FF, by elution with 20mM sodium phosphate buffer pH 7.5, containing 0.1M NaCl. The eluted mAb was directly loaded onto an immobilized metal affinity chromatography column (IMAC, Zn(2+)-iminodiacetic acid-Sepharose 6B) and eluted by stepwise pH gradient. The proposed method offered 162-fold purification with 97.2% purity and 63% yield. Analysis of the antibody preparation by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), enzyme immunosorbent assay (ELISA) and western blot showed that the mAb 2G12 was fully active and free of degraded variants, polyphenols and alkaloids. The effectiveness of the present purification protocol was evaluated by using a second transgenic human monoclonal anti-HIV mAb 4E10. The results showed that the same procedure can be successfully used for the purification of mAb 4E10. In the case of mAb 4E10, the proposed method offered 148-fold purification with 96.2% purity and 36% yield. Therefore, the proposed protocol may be of generic use for the purification of mAbs from transgenic tobacco plants.

Recent progress in the development of plant derived vaccines

Recombinant subunit vaccines have been with us for the last 30 years and they provide us with the unique opportunity to choose from the many available production systems that can be used for recombinant protein expression. Plants have become an attractive production platform for recombinant biopharmaceuticals and vaccines have been at the forefront of this new and expanding industry sector. The particular advantages of plant-based vaccines in terms of cost, safety and scalability are discussed in the light of recent successful clinical trials and the likely impact of plant systems on the vaccine industry is evaluated.

Extremely high-level and rapid transient protein production in plants without the use of viral replication

Plant-based overexpression of heterologous proteins has attracted much interest and development in recent years. To date, the most efficient vectors have been based on RNA virus-derived replicons. A system based on a disabled version of cowpea mosaic virus RNA-2 has been developed, which overcomes limitations on insert size and introduces biocontainment. This system involves positioning a gene of interest between the 5' leader sequence and 3' untranslated region (UTR) of RNA-2, thereby emulating a presumably stable mRNA for efficient translation. Thus far, the sequence of the 5' UTR has been preserved to maintain the ability of the modified RNA-2 to be replicated by RNA-1. However, high-level expression may be achieved in the absence of RNA-1-derived replication functions using Agrobacterium-mediated transient transformation. To investigate those features of the 5' UTR necessary for efficient expression, we have addressed the role of two AUG codons found within the 5' leader sequence upstream of the main initiation start site. Deletion of an in-frame start codon upstream of the main translation initiation site led to a massive increase in foreign protein accumulation. By 6 d postinfiltration, a number of unrelated proteins, including a full-size IgG and a self-assembling virus-like particle, were expressed to >10% and 20% of total extractable protein, respectively. Thus, this system provides an ideal vehicle for high-level expression that does not rely on viral replication of transcripts.

Antibody-mediated prevention of Fusarium mycotoxins in the field

Fusarium mycotoxins directly accumulated in grains during the infection of wheat and other cereal crops by Fusarium head blight (FHB) pathogens are detrimental to humans and domesticated animals. Prevention of the mycotoxins via the development of FHB-resistant varieties has been a challenge due to the scarcity of natural resistance against FHB pathogens. Various antibodies specific to Fusarium fungi and mycotoxins are widely used in immunoassays and antibody-mediated resistance in planta against Fusarium pathogens has been demonstrated. Antibodies fused to antifungal proteins have been shown to confer a very significantly enhanced Fusarium resistance in transgenic plants. Thus, antibody fusions hold great promise as an effective tool for the prevention of mycotoxin contaminations in cereal grains. This review highlights the utilization of protective antibodies derived from phage display to increase endogenous resistance of wheat to FHB pathogens and consequently to reduce mycotoxins in field. The role played by Fusarium-specific antibody in the resistance is also discussed.

Improvement of recombinant hGM-CSF production by suppression of cysteine proteinase gene expression using RNA interference in a transgenic rice culture

Recombinant proteins have been previously synthesized in a transgenic rice cell suspension culture system with the rice amylase 3D promoter, which can be induced via sugar starvation. However, the secreted recombinant proteins have been shown to be rapidly decreased as the result of proteolytic degradation occurring during prolonged incubation. The secreted proteases were identified via two-dimensional electrophoresis (2-DE) and ESI/Q-TOF mass spectrometry analyses. The internal amino acid sequences of 8 of 37 spots corresponded to cysteine proteinase (CysP), which is encoded for by Rep1 and EP3A. This result shows that CysP is a major secreted protease in rice cell suspension cultures following induction via sugar starvation. Intron-containing self-complementary hairpin RNA (ihpRNA)-mediated post-transcriptional gene silencing (PTGS) was applied to suppress the expression of CysP in rice cell suspension cultures. The reduction of rice CysP mRNA and the detection of siRNA specific to CysP, an initiator of RNAi, were verified via Northern blot analysis and RNase protection assays, respectively, thereby indicating that PTGS operated successfully in this system. The analysis of total secreted protease and CysP activities evidenced lower activity than was observed with the wild-type. Furthermore, suspension cultures of rice cells transformed with both hGM-CSF and the gene expressing the ihpRNA of CysP evidenced a reduction in total protease and CysP activities, and an up to 1.9-fold improvement in hGM-CSF production as compared to that observed in a rice cell line expressing hGM-CSF only. These results demonstrate the feasibility of the suppression of CysP via RNA interference to reduce protease activity and to increase target protein accumulation in rice cell suspension cultures.

Preventing unintended proteolysis in plant protein biofactories

Numerous reports have been published over the last decade assessing the potential of plants as useful hosts for the heterologous expression of clinically useful proteins. Significant progress has been made, in particular, in optimizing transgene transcription and translation in plants, and in elucidating the complex post-translational modifications of proteins typical of the plant cell machinery. In this article, we address the important issue of recombinant protein degradation in plant expression platforms, which directly impacts on the final yield, homogeneity and overall quality of the resulting protein product. Unlike several more stable and structurally less complex pharmaceuticals, recombinant proteins present a natural tendency to structural heterogeneity, resulting in part from the inherent instability of polypeptide chains expressed in heterologous environments. Proteolytic processing, notably, may dramatically alter the structural integrity and overall accumulation of recombinant proteins in plant expression systems, both in planta during expression and ex planta after extraction. In this article, we describe the current strategies proposed to minimize protein hydrolysis in plant protein factories, including organ-specific transgene expression, organelle-specific protein targeting, the grafting of stabilizing protein domains to labile proteins, protein secretion in natural fluids and the co-expression of companion protease inhibitors.

Efficient expression of a Toxoplasma gondii dense granule Gra4 antigen in tobacco leaves

A His-tagged truncated version of Toxoplasma gondii dense granule 4 protein (Gra4(163-345)) was transiently expressed in tobacco leaves. Two genetic constructions were used to accomplish this goal. In one of them, based in a Potato virus X (PVX) amplicon, the sequence encoding His-Gra4(163-345) was placed under control of an additional PVX coat protein subgenomic promoter. In the other, the same sequence was fused to an apoplastic transport signal and placed under the direction of the cauliflower mosaic virus 35S promoter. His-Gra4(163-345) accumulation in agroinfiltrated tobacco leaves was estimated by Western blot analysis using mouse anti-Gra4 antibody and a seropositive human serum. Here, we demonstrated the feasibility of producing a Gra4 antigen using transient expression methods in plants.