The Engineered Chloroplast Genome Just Got Smarter

Chloroplasts are known to sustain life on earth by providing food, fuel, and oxygen through the process of photosynthesis. However, the chloroplast genome has also been smartly engineered to confer valuable agronomic traits and/or serve as bioreactors for the production of industrial enzymes, biopharmaceuticals, bioproducts, or vaccines. The recent breakthrough in hyperexpression of biopharmaceuticals in edible leaves has facilitated progression to clinical studies by major pharmaceutical companies. This review critically evaluates progress in developing new tools to enhance or simplify expression of targeted genes in chloroplasts. These tools hold the promise to further the development of novel fuels and products, enhance the photosynthetic process, and increase our understanding of retrograde signaling and cellular processes.

Plant-made oral vaccines against human infectious diseases-Are we there yet?

Although the plant-made vaccine field started three decades ago with the promise of developing low-cost vaccines to prevent infectious disease outbreaks and epidemics around the globe, this goal has not yet been achieved. Plants offer several major advantages in vaccine generation, including low-cost production by eliminating expensive fermentation and purification systems, sterile delivery and cold storage/transportation. Most importantly, oral vaccination using plant-made antigens confers both mucosal (IgA) and systemic (IgG) immunity. Studies in the past 5 years have made significant progress in expressing vaccine antigens in edible leaves (especially lettuce), processing leaves or seeds through lyophilization and achieving antigen stability and efficacy after prolonged storage at ambient temperatures. Bioencapsulation of antigens in plant cells protects them from the digestive system; the fusion of antigens to transmucosal carriers enhances efficiency of their delivery to the immune system and facilitates successful development of plant vaccines as oral boosters. However, the lack of oral priming approaches diminishes these advantages because purified antigens, cold storage/transportation and limited shelf life are still major challenges for priming with adjuvants and for antigen delivery by injection. Yet another challenge is the risk of inducing tolerance without priming the host immune system. Therefore, mechanistic aspects of these two opposing processes (antibody production or suppression) are discussed in this review. In addition, we summarize recent progress made in oral delivery of vaccine antigens expressed in plant cells via the chloroplast or nuclear genomes and potential challenges in achieving immunity against infectious diseases using cold-chain-free vaccine delivery approaches.

When plant virology met Agrobacterium: the rise of the deconstructed clones

In the early days of molecular farming, Agrobacterium-mediated stable genetic transformation and the use of plant virus-based vectors were considered separate and competing technologies with complementary strengths and weaknesses. The demonstration that 'agroinfection' was the most efficient way of delivering virus-based vectors to their target plants blurred the distinction between the two technologies and permitted the development of 'deconstructed' vectors based on a number of plant viruses. The tobamoviruses, potexviruses, tobraviruses, geminiviruses and comoviruses have all been shown to be particularly well suited to the development of such vectors in dicotyledonous plants, while the development of equivalent vectors for use in monocotyledonous plants has lagged behind. Deconstructed viral vectors have proved extremely effective at the rapid, high-level production of a number of pharmaceutical proteins, some of which are currently undergoing clinical evaluation.

Plant-produced viral bovine vaccines: what happened during the last 10 years?

Vaccination has proved to be an efficient strategy to deal with viral infections in both human and animal species. However, protection of cattle against viral infections is still a major concern in veterinary science. During the last two decades, the development of efficient plant-based expression strategies for recombinant proteins prompted the application of this methodology for veterinary vaccine purposes. The main goals of viral bovine vaccines are to improve the health and welfare of cattle and increase the production of livestock, in a cost-effective manner. This review explores some of the more prominent recent advances in plant-made viral bovine vaccines against foot-and-mouth disease virus (FMDV), bovine rotavirus (BRV), bovine viral diarrhoea virus (BVDV), bluetongue virus (BTV) and bovine papillomavirus (BPV), some of which are considered to be the most important viral causative agents of economic loss in cattle production.

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

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

Antibody proteolysis: a common picture emerging from plants

We have recently characterized the degradation profiles of 2 human IgG1 monoclonal antibodies, the tumor-targeting mAb H10 and the anti-HIV mAb 2G12. Both mAbs were produced in plants either as stable transgenics or using a transient expression system based on leaf agroinfiltration. The purified antibodies were separated by 1DE and protein bands were characterized by N-terminal sequencing. The proteolytic cleavage sites identified in the heavy chain (HC) of both antibodies were localized in 3 inter-domain regions, suggesting that the number of proteolytic cleavage events taking place in plants is limited. One of the cleavage sites, close to the hinge region, was common to both antibodies.

Site-specific proteolytic degradation of IgG monoclonal antibodies expressed in tobacco plants

Plants are promising hosts for the production of monoclonal antibodies (mAbs). However, proteolytic degradation of antibodies produced both in stable transgenic plants and using transient expression systems is still a major issue for efficient high-yield recombinant protein accumulation. In this work, we have performed a detailed study of the degradation profiles of two human IgG1 mAbs produced in plants: an anti-HIV mAb 2G12 and a tumour-targeting mAb H10. Even though they use different light chains (κ and λ, respectively), the fragmentation pattern of both antibodies was similar. The majority of Ig fragments result from proteolytic degradation, but there are only a limited number of plant proteolytic cleavage events in the immunoglobulin light and heavy chains. All of the cleavage sites identified were in the proximity of interdomain regions and occurred at each interdomain site, with the exception of the VL /CL interface in mAb H10 λ light chain. Cleavage site sequences were analysed, and residue patterns characteristic of proteolytic enzymes substrates were identified. The results of this work help to define common degradation events in plant-produced mAbs and raise the possibility of predicting antibody degradation patterns 'a priori' and designing novel stabilization strategies by site-specific mutagenesis.

Plant-derived SAC domain of PAR-4 (Prostate Apoptosis Response 4) exhibits growth inhibitory effects in prostate cancer cells

The gene Par-4 (Prostate Apoptosis Response 4) was originally identified in prostate cancer cells undergoing apoptosis and its product Par-4 showed cancer specific pro-apoptotic activity. Particularly, the SAC domain of Par-4 (SAC-Par-4) selectively kills cancer cells leaving normal cells unaffected. The therapeutic significance of bioactive SAC-Par-4 is enormous in cancer biology; however, its large scale production is still a matter of concern. Here we report the production of SAC-Par-4-GFP fusion protein coupled to translational enhancer sequence (5' AMV) and apoplast signal peptide (aTP) in transgenic Nicotiana tabacum cv. Samsun NN plants under the control of a unique recombinant promoter M24. Transgene integration was confirmed by genomic DNA PCR, Southern and Northern blotting, Real-time PCR, and Nuclear run-on assays. Results of Western blot analysis and ELISA confirmed expression of recombinant SAC-Par-4-GFP protein and it was as high as 0.15% of total soluble protein. In addition, we found that targeting of plant recombinant SAC-Par-4-GFP to the apoplast and endoplasmic reticulum (ER) was essential for the stability of plant recombinant protein in comparison to the bacterial derived SAC-Par-4. Deglycosylation analysis demonstrated that ER-targeted SAC-Par-4-GFP-SEKDEL undergoes O-linked glycosylation unlike apoplast-targeted SAC-Par-4-GFP. Furthermore, various in vitro studies like mammalian cells proliferation assay (MTT), apoptosis induction assays, and NF-κB suppression suggested the cytotoxic and apoptotic properties of plant-derived SAC-Par-4-GFP against multiple prostate cancer cell lines. Additionally, pre-treatment of MAT-LyLu prostate cancer cells with purified SAC-Par-4-GFP significantly delayed the onset of tumor in a syngeneic rat prostate cancer model. Taken altogether, we proclaim that plant made SAC-Par-4 may become a useful alternate therapy for effectively alleviating cancer in the new era.

Engineering plastid genomes: methods, tools, and applications in basic research and biotechnology

The small bacterial-type genome of the plastid (chloroplast) can be engineered by genetic transformation, generating cells and plants with transgenic plastid genomes, also referred to as transplastomic plants. The transformation process relies on homologous recombination, thereby facilitating the site-specific alteration of endogenous plastid genes as well as the precisely targeted insertion of foreign genes into the plastid DNA. The technology has been used extensively to analyze chloroplast gene functions and study plastid gene expression at all levels in vivo. Over the years, a large toolbox has been assembled that is now nearly comparable to the techniques available for plant nuclear transformation and that has enabled new applications of transplastomic technology in basic and applied research. This review describes the state of the art in engineering the plastid genomes of algae and land plants (Embryophyta). It provides an overview of the existing tools for plastid genome engineering, discusses current technological limitations, and highlights selected applications that demonstrate the immense potential of chloroplast transformation in several key areas of plant biotechnology.

The Induction of Recombinant Protein Bodies in Different Subcellular Compartments Reveals a Cryptic Plastid-Targeting Signal in the 27-kDa γ-Zein Sequence

Naturally occurring storage proteins such as zeins are used as fusion partners for recombinant proteins because they induce the formation of ectopic storage organelles known as protein bodies (PBs) where the proteins are stabilized by intermolecular interactions and the formation of disulfide bonds. Endogenous PBs are derived from the endoplasmic reticulum (ER). Here, we have used different targeting sequences to determine whether ectopic PBs composed of the N-terminal portion of mature 27 kDa γ-zein added to a fluorescent protein could be induced to form elsewhere in the cell. The addition of a transit peptide for targeting to plastids causes PB formation in the stroma, whereas in the absence of any added targeting sequence PBs were typically associated with the plastid envelope, revealing the presence of a cryptic plastid-targeting signal within the γ-zein cysteine-rich domain. The subcellular localization of the PBs influences their morphology and the solubility of the stored recombinant fusion protein. Our results indicate that the biogenesis and budding of PBs does not require ER-specific factors and therefore, confirm that γ-zein is a versatile fusion partner for recombinant proteins offering unique opportunities for the accumulation and bioencapsulation of recombinant proteins in different subcellular compartments.

Oral delivery of Angiotensin-converting enzyme 2 and Angiotensin-(1-7) bioencapsulated in plant cells attenuates pulmonary hypertension

Emerging evidences indicate that diminished activity of the vasoprotective axis of the renin-angiotensin system, constituting angiotensin-converting enzyme 2 (ACE2) and its enzymatic product, angiotensin-(1-7) [Ang-(1-7)] contribute to the pathogenesis of pulmonary hypertension (PH). However, long-term repetitive delivery of ACE2 or Ang-(1-7) would require enhanced protein stability and ease of administration to improve patient compliance. Chloroplast expression of therapeutic proteins enables their bioencapsulation within plant cells to protect against gastric enzymatic degradation and facilitates long-term storage at room temperature. Besides, fusion to a transmucosal carrier helps effective systemic absorption from the intestine on oral delivery. We hypothesized that bioencapsulating ACE2 or Ang-(1-7) fused to the cholera nontoxin B subunit would enable development of an oral delivery system that is effective in treating PH. PH was induced in male Sprague Dawley rats by monocrotaline administration. Subset of animals was simultaneously treated with bioencapsulaed ACE2 or Ang-(1-7) (prevention protocol). In a separate set of experiments, drug treatment was initiated after 2 weeks of PH induction (reversal protocol). Oral feeding of rats with bioencapsulated ACE2 or Ang-(1-7) prevented the development of monocrotaline-induced PH and improved associated cardiopulmonary pathophysiology. Furthermore, in the reversal protocol, oral ACE2 or Ang-(1-7) treatment significantly arrested disease progression, along with improvement in right heart function, and decrease in pulmonary vessel wall thickness. In addition, a combination therapy with ACE2 and Ang-(1-7) augmented the beneficial effects against monocrotaline-induced lung injury. Our study provides proof-of-concept for a novel low-cost oral ACE2 or Ang-(1-7) delivery system using transplastomic technology for pulmonary disease therapeutics.

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

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

Molecular farming in tobacco hairy roots by triggering the secretion of a pharmaceutical antibody

Recombinant pharmaceutical proteins expressed in hairy root cultures can be secreted into the medium to improve product homogeneity and to facilitate purification, although this may result in significant degradation if the protein is inherently unstable or particularly susceptible to proteases. To address these challenges, we used a design of experiments approach to develop an optimized induction protocol for the cultivation of tobacco hairy roots secreting the full-size monoclonal antibody M12. The antibody yield was enhanced 30-fold by the addition of 14 g/L KNO3 , 19 mg/L 1-naphthaleneacetic acid and 1.5 g/L of the stabilizing agent polyvinylpyrrolidone. Analysis of hairy root cross sections revealed that the optimized medium induced lateral root formation and morphological changes in the inner cortex and pericycle cells, indicating that the improved productivity was at least partially based on the enhanced efficiency of antibody secretion. We found that 57% of the antibody was secreted, yielding 5.9 mg of product per liter of induction medium. Both the secreted and intracellular forms of the antibody could be isolated by protein A affinity chromatography and their functionality was confirmed using vitronectin-binding assays. Glycan analysis revealed three major plant complex-type glycans on both forms of the antibody, although the secreted form was more homogeneous due to the predominance of a specific glycoform. Tobacco hairy root cultures therefore offer a practical solution for the production of homogeneous pharmaceutical antibodies in containment.

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

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

The production of recombinant cationic α-helical antimicrobial peptides in plant cells induces the formation of protein bodies derived from the endoplasmic reticulum

Synthetic linear antimicrobial peptides with cationic α-helical structures, such as BP100, are valuable as novel therapeutics and preservatives. However, they tend to be toxic when expressed at high levels as recombinant peptides in plants, and they can be difficult to detect and isolate from complex plant tissues because they are strongly cationic and display low extinction coefficient and extremely limited immunogenicity. We therefore expressed BP100 with a C-terminal tag which preserved its antimicrobial activity and demonstrated significant accumulation in plant cells. We used a fluorescent tag to trace BP100 following transiently expression in Nicotiana benthamiana leaves and showed that it accumulated in large vesicles derived from the endoplasmic reticulum (ER) along with typical ER luminal proteins. Interestingly, the formation of these vesicles was induced by BP100. Similar vesicles formed in stably transformed Arabidopsis thaliana seedlings, but the recombinant peptide was toxic to the host during latter developmental stages. This was avoided by selecting active BP100 derivatives based on their low haemolytic activity even though the selected peptides remained toxic to plant cells when applied exogenously at high doses. Using this strategy, we generated transgenic rice lines producing active BP100 derivatives with a yield of up to 0.5% total soluble protein.

Characterization of a plant-produced recombinant human secretory IgA with broad neutralizing activity against HIV

Recombinant Secretory IgA (SIgA) complexes have the potential to improve antibody-based passive immunotherapeutic approaches to combat many mucosal pathogens. In this report, we describe the expression, purification and characterization of a human SIgA format of the broadly neutralizing anti-HIV monoclonal antibody (mAb) 2G12, using both transgenic tobacco plants and transient expression in Nicotiana benthamiana as expression hosts (P2G12 SIgA). The resulting heterodecameric complexes accumulated in intracellular compartments in leaf tissue, including the vacuole. SIgA complexes could not be detected in the apoplast. Maximum yields of antibody were 15.2 μg/g leaf fresh mass (LFM) in transgenic tobacco and 25 μg/g LFM after transient expression, and assembly of SIgA complexes was superior in transgenic tobacco. Protein L purified antibody specifically bound HIV gp140 and neutralised tier 2 and tier 3 HIV isolates. Glycoanalysis revealed predominantly high mannose structures present on most N-glycosylation sites, with limited evidence for complex glycosylation or processing to paucimannosidic forms. O-glycan structures were not identified. Functionally, P2G12 SIgA, but not IgG, effectively aggregated HIV virions. Binding of P2G12 SIgA was observed to CD209 / DC-SIGN, but not to CD89 / FcalphaR on a monocyte cell line. Furthermore, P2G12 SIgA demonstrated enhanced stability in mucosal secretions in comparison to P2G12 IgG mAb.

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

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

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

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

Optimization of BY-2 cell suspension culture medium for the production of a human antibody using a combination of fractional factorial designs and the response surface method

We have developed a strategy for the optimization of plant cell suspension culture media using a combination of fractional factorial designs (FFDs) and response surface methodology (RSM). This sequential approach was applied to transformed tobacco BY-2 cells secreting a human antibody (M12) into the culture medium, in an effort to maximize yields. We found that the nutrients KNO₃, NH₄NO₃ and CaCl₂ and the hormones 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-benzylaminopurine (BAP) had the most significant impact on antibody accumulation. The factorial screening revealed strong interactions within the nutrients group (KNO₃, NH₄NO₃ and CaCl₂) and also individually between 2,4-D and three other components (KNO₃, NH₄NO₃ and BAP). The RSM design resulted in a fivefold increase in the antibody concentration after 5 days and a twofold reduction in the packed cell volume (PCV). Longer cultivation in the optimized medium led to the further accumulation of antibody M12 in the culture medium (up to 107 μg/mL, day 10). Because the packed cell volume was reduced in the optimized medium, this enhanced the overall yield by 20-fold (day 7) and 31-fold (day 10) compared to the conventional MS medium.

Engineering of human-type O-glycosylation in Nicotiana benthamiana plants

Therapeutic properties of recombinant proteins are very often affected by the composition and heterogeneity of their glycans. Conventional expression systems for recombinant pharmaceutical proteins typically do not address this problem and produce a mixture of glycoforms that are neither identical to human glycans nor optimized for enhanced efficacy. In terms of glycosylation, plants offer certain advantages over mammalian cells as the N-glycosylation pathway of plants is comparably simple and a typical mammalian O-glycosylation pathway is not present at all. During the last ten years we have developed a plant-based expression platform for the generation of recombinant glycoproteins with defined N-glycans. Now we have extended our tool-box for glyco-engineering in the tobacco related species Nicotiana benthamiana toward the production of tailored mucin-type O-glycans on recombinant proteins.

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

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

Oral delivery of human biopharmaceuticals, autoantigens and vaccine antigens bioencapsulated in plant cells

Among 12billion injections administered annually, unsafe delivery leads to >20million infections and >100million reactions. In an emerging new concept, freeze-dried plant cells (lettuce) expressing vaccine antigens/biopharmaceuticals are protected in the stomach from acids/enzymes but are released to the immune or blood circulatory system when plant cell walls are digested by microbes that colonize the gut. Vaccine antigens bioencapsulated in plant cells upon oral delivery after priming, conferred both mucosal and systemic immunity and protection against bacterial, viral or protozoan pathogens or toxin challenge. Oral delivery of autoantigens was effective against complications of type 1 diabetes and hemophilia, by developing tolerance. Oral delivery of proinsulin or exendin-4 expressed in plant cells regulated blood glucose levels similar to injections. Therefore, this new platform offers a low cost alternative to deliver different therapeutic proteins to combat infectious or inherited diseases by eliminating inactivated pathogens, expensive purification, cold storage/transportation and sterile injections.

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

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

Protein body formation in stable transgenic tobacco expressing elastin-like polypeptide and hydrophobin fusion proteins

BACKGROUND

Plants are recognized as an efficient and inexpensive system to produce valuable recombinant proteins. Two different strategies have been commonly used for the expression of recombinant proteins in plants: transient expression mediated by Agrobacterium; or stable transformation of the plant genome. However, the use of plants as bioreactors still faces two main limitations: low accumulation levels of some recombinant proteins and lack of efficient purification methods. Elastin-like polypeptide (ELP), hydrophobin I (HFBI) and Zera® are three fusion partners found to increase the accumulation levels of recombinant proteins and induce the formation of protein bodies (PBs) in leaves when targeted to the endoplasmic reticulum (ER) in transient expression assays. In this study the effects of ELP and HFBI fusion tags on recombinant protein accumulation levels and PB formation was examined in stable transgenic Nicotiana tabacum.

RESULTS

The accumulation of recombinant protein and PB formation was evaluated in two cultivars of Nicotiana tabacum transformed with green fluorescent protein (GFP) fused to ELP or HFBI, both targeted and retrieved to the ER. The ELP and HFBI tags increased the accumulation of the recombinant protein and induced the formation of PBs in leaves of stable transgenic plants from both cultivars. Furthermore, these tags induced the formation of PBs in a concentration-dependent manner, where a specific level of recombinant protein accumulation was required for PBs to appear. Moreover, agro-infiltration of plants accumulating low levels of recombinant protein with p19, a suppressor of post-transcriptional gene silencing (PTGS), increased accumulation levels in four independent transgenic lines, suggesting that PTGS might have caused the low accumulation levels in these plants.

CONCLUSION

The use of ELP and HFBI tags as fusion partners in stable transgenic plants of tobacco is feasible and promising. In a constitutive environment, these tags increase the accumulation levels of the recombinant protein and induce the formation of PBs regardless of the cultivar used. However, a specific level of recombinant protein accumulation needs to be reached for PBs to form.

Increasing the production yield of recombinant protein in transgenic seeds by expanding the deposition space within the intracellular compartment

Seeds must maintain a constant level of nitrogen in order to germinate. When recombinant proteins are produced while endogenous seed protein expression is suppressed, the production levels of the foreign proteins increase to compensate for the decreased synthesis of endogenous proteins. Thus, exchanging the production of endogenous seed proteins for that of foreign proteins is a promising approach to increase the yield of foreign recombinant proteins. Providing a space for the deposition of recombinant protein in the intracellular compartment is critical, at this would lessen any competition in this region between the endogenous seed proteins and the introduced foreign protein. The production yields of several recombinant proteins have been greatly increased by this strategy.

Oral delivery of bioencapsulated exendin-4 expressed in chloroplasts lowers blood glucose level in mice and stimulates insulin secretion in beta-TC6 cells

Glucagon-like peptide (GLP-1) increases insulin secretion but is rapidly degraded (half-life: 2 min in circulation). GLP-1 analogue, exenatide (Byetta) has a longer half-life (3.3-4 h) with potent insulinotropic effects but requires cold storage, daily abdominal injections with short shelf life. Because patients with diabetes take >60 000 injections in their life time, alternative delivery methods are highly desired. Exenatide is ideal for oral delivery because insulinotropism is glucose dependent, with reduced risk of hypoglycaemia even at higher doses. Therefore, exendin-4 (EX4) was expressed as a cholera toxin B subunit (CTB)-fusion protein in tobacco chloroplasts to facilitate bioencapsulation within plant cells and transmucosal delivery in the gut via GM1 receptors present in the intestinal epithelium. The transgene integration was confirmed by PCR and Southern blot analysis. Expression level of CTB-EX4 reached up to 14.3% of total leaf protein (TLP). Lyophilization of leaf material increased therapeutic protein concentration by 12- to 24-fold, extended their shelf life up to 15 months when stored at room temperature and eliminated microbes present in fresh leaves. The pentameric structure, disulphide bonds and functionality of CTB-EX4 were well preserved in lyophilized materials. Chloroplast-derived CTB-EX4 showed increased insulin secretion similar to the commercial EX4 in beta-TC6, a mouse pancreatic cell line. Even when 5000-fold excess dose of CTB-EX4 was orally delivered, it stimulated insulin secretion similar to the intraperitoneal injection of commercial EX4 but did not cause hypoglycaemia in mice. Oral delivery of the bioencapsulated EX4 should eliminate injections, increase patient compliance/convenience and significantly lower their cost.

Endosperm-specific expression of human acid beta-glucosidase in a waxy rice

BACKGROUND

The deficiency of human acid beta-glucosidase (hGCase) causes Gaucher disease, a rare genetically-inherited disorder currently treated by enzyme replacement therapy using recombinant CHO-derived GCase. In an attempt to provide an alternative and more efficient production system, a chimeric cDNA coding for hGCase operatively linked to the signal peptide of rice glutelin 4 (GluB4) was put under the control of the GluB4 endosperm-specific promoter and inserted into the genome of a waxy rice.

RESULTS

Molecular, immunological and biochemical analyses showed that recombinant hGCase, targeted to the protein storage vacuoles of rice endosperm cells, is equivalent to the native protein and has a glycosylation pattern compatible with direct therapeutic use. Compared to a previous study carried out on transgenic tobacco seeds, enzyme contents per unit of biomass were drastically increased; in addition, differently from what observed in tobacco, rice seed viability was unaffected by hGCase even at the highest production level. Transgenic seed polishing combined with a pretreatment of seed flour greatly facilitated hGCase extraction and purification with an industrially-scalable procedure.

CONCLUSIONS

This study opens up the possibility to efficiently produce in the rice seed pharmaceutical compounds which are available in limited amounts or completely excluded from clinical practice due to the inadequacy of their production systems.

Unsolved problems in plastid transformation

Plants have been proved as a novel production platform for a wide range of biologically important compounds such as enzymes, therapeutic proteins, antibiotics, and proteins with immunological properties. In this context, plastid genetic engineering can be potentially used to produce recombinant proteins. However, several challenges still remain to be overcome if the full potential of plastid transformation technology is to be realized. They include the development of plastid transformation systems for species other than tobacco, the expression of transgenes in non-green plastids, the increase of protein accumulation and the appearance of pleiotropic effects. In this paper, we discuss the novel tools recently developed to overcome some limitations of chloroplast transformation.

Challenges in O-glycan engineering of plants

Plants are attractive alternative expression hosts for the production of recombinant proteins. Many therapeutic proteins are glycosylated with N- and O-glycosylation being the most prevalent forms of protein glycosylation. While N-glycans have already been modified in plants toward the formation of homogenous mammalian-type glycoforms with equal or improved biological function compared to mammalian-cell culture produced glycoproteins little attention has been paid to the modification of O-linked glycans. Recently, the first step of mammalian O-glycan biosynthesis has been accomplished in plants. However, as outlined in this short review there are important issues that have to be addressed in the future. These include: (i) elimination of potentially immunogenic or allergenic carbohydrate epitopes containing arabinosides or arabinogalactans, (ii) a detailed investigation of the interplay between engineered N- and O-glycosylation pathways to avoid competition for common metabolites like UDP-GlcNAc, and (iii) a deeper understanding of signals and mechanisms for distribution of glycan processing enzymes, which is a prerequisite for complete and homogenous glycosylation of recombinant proteins.

Transient co-expression of post-transcriptional gene silencing suppressors for increased in planta expression of a recombinant anthrax receptor fusion protein

Potential epidemics of infectious diseases and the constant threat of bioterrorism demand rapid, scalable, and cost-efficient manufacturing of therapeutic proteins. Molecular farming of tobacco plants provides an alternative for the recombinant production of therapeutics. We have developed a transient production platform that uses Agrobacterium infiltration of Nicotiana benthamiana plants to express a novel anthrax receptor decoy protein (immunoadhesin), CMG2-Fc. This chimeric fusion protein, designed to protect against the deadly anthrax toxins, is composed of the von Willebrand factor A (VWA) domain of human capillary morphogenesis 2 (CMG2), an effective anthrax toxin receptor, and the Fc region of human immunoglobulin G (IgG). We evaluated, in N. benthamiana intact plants and detached leaves, the expression of CMG2-Fc under the control of the constitutive CaMV 35S promoter, and the co-expression of CMG2-Fc with nine different viral suppressors of post-transcriptional gene silencing (PTGS): p1, p10, p19, p21, p24, p25, p38, 2b, and HCPro. Overall, transient CMG2-Fc expression was higher on intact plants than detached leaves. Maximum expression was observed with p1 co-expression at 3.5 days post-infiltration (DPI), with a level of 0.56 g CMG2-Fc per kg of leaf fresh weight and 1.5% of the total soluble protein, a ten-fold increase in expression when compared to absence of suppression. Co-expression with the p25 PTGS suppressor also significantly increased the CMG2-Fc expression level after just 3.5 DPI.

A trial of production of the plant-derived high-value protein in a plant factory: photosynthetic photon fluxes affect the accumulation of recombinant miraculin in transgenic tomato fruits

One of the ultimate goals of plant science is to test a hypothesis obtained by basic science and to apply it to agriculture and industry. A plant factory is one of the ideal systems for this trial. Environmental factors affect both plant yield and the accumulation of recombinant proteins for industrial applications within transgenic plants. However, there have been few reports studying plant productivity for recombinant protein in closed cultivation systems called plant factories. To investigate the effects of photosynthetic photon flux (PPF) on tomato fruit yield and the accumulation of recombinant miraculin, a taste-modifying glycoprotein, in transgenic tomato fruits, plants were cultivated at various PPFs from 100 to 400 (µmol m(-2) s(-)1) in a plant factory. Miraculin production per unit of energy used was highest at PPF100, although miraculin production per unit area was highest at PPF300. The commercial productivity of recombinant miraculin in transgenic tomato fruits largely depended on light conditions in the plant factory. Our trial will be useful to consider the trade-offs between the profits from production of high-value materials in plants and the costs of electricity.

Expression of dengue-3 premembrane and envelope polyprotein in lettuce chloroplasts

Dengue is an acute febrile viral disease with >100 million infections occurring each year and more than half of the world population is at risk. Global resurgence of dengue in many urban centers of the tropics is a major concern. Therefore, development of a successful vaccine is urgently needed that is economical and provide long-lasting protection from dengue virus infections. In this manuscript, we report expression of dengue-3 serotype polyprotein (prM/E) consisting of part of capsid, complete premembrane (prM) and truncated envelope (E) protein in an edible crop lettuce. The dengue sequence was controlled by endogenous Lactuca sativa psbA regulatory elements. PCR and Southern blot analysis confirmed transgene integration into the lettuce chloroplast genome via homologous recombination at the trnI/trnA intergenic spacer region. Western blot analysis showed expression of polyprotein prM/E in different forms as monomers (~65 kDa) or possibly heterodimers (~130 kDa) or multimers. Multimers were solubilized into monomers using guanidine hydrochloride. Transplastomic lettuce plants expressing dengue prM/E vaccine antigens grew normally and transgenes were inherited in the T1 progeny without any segregation. Transmission electron microscopy showed the presence of virus-like particles of ~20 nm diameter in chloroplast extracts of transplastomic lettuce expressing prM/E proteins, but not in untransformed plants. The prM/E antigens expressed in lettuce chloroplasts should offer a potential source for investigating an oral Dengue vaccine.

Recombinant cytokines from plants

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

Low-cost production of proinsulin in tobacco and lettuce chloroplasts for injectable or oral delivery of functional insulin and C-peptide

Current treatment for type I diabetes includes delivery of insulin via injection or pump, which is highly invasive and expensive. The production of chloroplast-derived proinsulin should reduce cost and facilitate oral delivery. Therefore, tobacco and lettuce chloroplasts were transformed with the cholera toxin B subunit fused with human proinsulin (A, B, C peptides) containing three furin cleavage sites (CTB-PFx3). Transplastomic lines were confirmed for site-specific integration of transgene and homoplasmy. Old tobacco leaves accumulated proinsulin up to 47% of total leaf protein (TLP). Old lettuce leaves accumulated proinsulin up to 53% TLP. Accumulation was so stable that up to ~40% proinsulin in TLP was observed even in senescent and dried lettuce leaves, facilitating their processing and storage in the field. Based on the yield of only monomers and dimers of proinsulin (3 mg/g leaf, a significant underestimation), with a 50% loss of protein during the purification process, one acre of tobacco could yield up to 20 million daily doses of insulin per year. Proinsulin from tobacco leaves was purified up to 98% using metal affinity chromatography without any His-tag. Furin protease cleaved insulin peptides in vitro. Oral delivery of unprocessed proinsulin bioencapsulated in plant cells or injectable delivery into mice showed reduction in blood glucose levels similar to processed commercial insulin. C-peptide should aid in long-term treatment of diabetic complications including stimulation of nerve and renal functions. Hyper-expression of functional proinsulin and exceptional stability in dehydrated leaves offer a low-cost platform for oral and injectable delivery of cleavable proinsulin.

Expression and characterization of antimicrobial peptides Retrocyclin-101 and Protegrin-1 in chloroplasts to control viral and bacterial infections

Retrocyclin-101 (RC101) and Protegrin-1 (PG1) are two important antimicrobial peptides that can be used as therapeutic agents against bacterial and/or viral infections, especially those caused by the HIV-1 or sexually transmitted bacteria. Because of their antimicrobial activity and complex secondary structures, they have not yet been produced in microbial systems and their chemical synthesis is prohibitively expensive. Therefore, we created chloroplast transformation vectors with the RC101 or PG1 coding sequence, fused with GFP to confer stability, furin or Factor Xa cleavage site to liberate the mature peptide from their fusion proteins and a His-tag to aid in their purification. Stable integration of RC101 into the tobacco chloroplast genome and homoplasmy were confirmed by Southern blots. RC101 and PG1 accumulated up to 32%-38% and 17%∼26% of the total soluble protein. Both RC101 and PG1 were cleaved from GFP by corresponding proteases in vitro, and Factor Xa-like protease activity was observed within chloroplasts. Confocal microscopy studies showed location of GFP fluorescence within chloroplasts. Organic extraction resulted in 10.6-fold higher yield of RC101 than purification by affinity chromatography using His-tag. In planta bioassays with Erwinia carotovora confirmed the antibacterial activity of RC101 and PG1 expressed in chloroplasts. RC101 transplastomic plants were resistant to tobacco mosaic virus infections, confirming antiviral activity. Because RC101 and PG1 have not yet been produced in other cell culture or microbial systems, chloroplasts can be used as bioreactors for producing these proteins. Adequate yield of purified antimicrobial peptides from transplastomic plants should facilitate further preclinical studies.

Mulberry improvements via plastid transformation and tissue culture engineering

The in vitro tissue culture and micropropagation studies for Morus spp., a pivotal sericulture plant, are well established. The rapid and reproducible in vitro response to plant growth regulator treatments has emerged as an essential complement of transformation studies for this plant species. A major area of study is the use of protoplast culture and fusion techniques where advantages to mulberry improvement can be applied. The advancements in genetic transformation of mulberry are reviewed, and a section on strategy for transforming plastids (chloroplasts) of mulberry is included. A role for mulberry in "molecular farming" is envisioned. The conclusions and future prospects for improvement of this economically important tree species are proposed.

Efficient Agrobacterium-based transient expression system for the production of biopharmaceuticals in plants

We have recently described an efficient transient expression system mediated by Agrobacterium tumefaciens for the production of HIV-1 Nef protein in Nicotiana benthamiana plants. In order to enhance the yield of recombinant protein we assayed the effect of three gene-silencing viral suppressor proteins (P25 of Potato Virus X, P19 of Artichoke Mottled Crinckle virus and Tomato Bushy Stunt virus) on Nef expression levels. Results demonstrated that AMCV-P19 gave the highest Nef yield (1.3% of total soluble protein) and that this effect was correlated to a remarkable decrease of Nef-specific small interfering RNAs (siRNAs) indicating an effective modulation of RNA silencing mechanisms. Here we report additional data on the production of different heterologous proteins including human immunoglobulin heavy and light chains and a virus coat protein that demonstrate the robustness of this co-agroinfiltration expression system boosted by the AMCV-P19 gene-silencing suppressor.

Tobacco, a highly efficient green bioreactor for production of therapeutic proteins

Molecular farming of pharmaceuticals in plants has the potential to provide almost unlimited amounts of recombinant proteins for use in disease diagnosis, prevention or treatment. Tobacco has been and will continue to be a major crop for molecular farming and offers several practical advantages over other crops. It produces significant leaf biomass, has high soluble protein content and is a non-food crop, minimizing the risk of food-chain contamination. This, combined with its flexibility and highly-efficient genetic transformation/regeneration, has made tobacco particularly well suited for plant-based production of biopharmaceutical products. The goal of this review is to provide an update on the use of tobacco for molecular farming of biopharmaceuticals as well the technologies developed to enhance protein production/purification/efficacy. We show that tobacco is a robust biological reactor with a multitude of applications and may hold the key to success in plant molecular farming.

Plastid production of protein antibiotics against pneumonia via a new strategy for high-level expression of antimicrobial proteins

Plastid transformation has become an attractive tool in biotechnology. Because of the prokaryotic nature of the plastid's gene expression machinery, expression elements (promoters and untranslated regions) that trigger high-level foreign protein accumulation in plastids usually also confer high expression in bacterial cloning hosts. This can cause problems, for example, when production of antimicrobial compounds is attempted. Their bactericidal activity can make the cloning of the corresponding genes in plastid transformation vectors impossible. Here, we report a general solution to this problem. We have designed a strategy (referred to as toxin shuttle) that allows the expression in plastids of proteins that are toxic to Escherichia coli. The strategy is based on blocking transcription in E. coli by bacterial transcription terminators upstream of the gene of interest, which subsequently are excised in planta by site-specific recombination. We demonstrate the applicability of the strategy by the high-level expression in plastids (to up to 30% of the plant's total soluble protein) of 2 phage-derived protein antibiotics that are toxic to E. coli. We also show that the plastid-produced antibiotics efficiently kill pathogenic strains of Streptococcus pneumoniae, the causative agent of pneumonia, thus providing a promising strategy for the production of next-generation antibiotics in plants.

Production of recombinant allergens in plants

A large percentage of allergenic proteins are of plant origin. Hence, plant-based expression systems are considered ideal for the recombinant production of certain allergens. First attempts to establish production of plant-derived allergens in plants focused on transient expression in Nicotiana benthamiana infected with recombinant viral vectors. Accordingly, allergens from birch and mugwort pollen, as well as from apple have been expressed in plants. Production of house dust mite allergens has been achieved by Agrobacterium-mediated transformation of tobacco plants. Beside the use of plants as production systems, other approaches have focused on the development of edible vaccines expressing allergens or epitopes thereof, which bypasses the need of allergen purification. The potential of this approach has been convincingly demonstrated for transgenic rice seeds expressing seven dominant human T cell epitopes derived from Japanese cedar pollen allergens. Parallel to efforts in developing recombinant-based diagnostic and therapeutic reagents, different gene-silencing approaches have been used to decrease the expression of allergenic proteins in allergen sources. In this way hypoallergenic ryegrass, soybean, rice, apple, and tomato were developed.

Phytochemical approach and bioanalytical strategy to develop chaperone-based medications

Currently, no pharmaceuticals for the etiological treatment of degenerative protein-misfolding diseases (e.g., ALS, Alzheimer's or prion diseases) are commercially available. In this technical note theoretical considerations and practical approaches concerning the development of chaperone-based medications from medicinal plants (e.g., Ginkgo biloba) are reviewed and discussed in detail. Phytochaperones and other agents isolated from medicinal plants are proposed to serve as the general basis of drug development in protein-misfolding diseases.

Aberrant localization and underglycosylation of highly accumulating single-chain Fv-Fc antibodies in transgenic Arabidopsis seeds

Production of high-value recombinant proteins in transgenic seeds is an attractive and economically feasible alternative to conventional systems based on mammalian cells and bacteria. In contrast to leaves, seeds allow high-level accumulation of recombinant proteins in a relatively small volume and a stable environment. We demonstrate that single-chain variable fragment (scFv)-Fc antibodies, with N-terminal signal sequence and C-terminal KDEL tag, can accumulate to very high levels as bivalent IgG-like antibodies in Arabidopsis thaliana seeds and illustrate that a plant-produced anti-hepatitis A virus scFv-Fc has similar antigen-binding and in vitro neutralizing activities as the corresponding full-length IgG. As expected, most scFv-Fc produced in seeds contained only oligomannose-type N-glycans, but, unexpectedly, 35-40% was never glycosylated. A portion of the scFv-Fc was found in endoplasmic reticulum (ER)-derived compartments delimited by ribosome-associated membranes. Additionally, consistent with the glycosylation data, large amounts of the recombinant protein were deposited in the periplasmic space, implying a direct transport from the ER to the periplasmic space between the plasma membrane and the cell wall. Aberrant localization of the ER chaperones calreticulin and binding protein (BiP) and the endogenous seed storage protein cruciferin in the periplasmic space suggests that overproduction of recombinant scFv-Fc disturbs normal ER retention and protein-sorting mechanisms in the secretory pathway.

New approaches for studying and exploiting an old protuberance, the plant trichome

BACKGROUND AND AIMS

Much recent study of plant trichomes has focused on various aspects of glandular secreting trichomes (GSTs) and differentiation of simple trichomes. This Botanical Briefing will highlight: research on various aspects of, and manipulation of glandular secreting trichomes; molecular aspects of the differentiation and development of simple trichomes of arabidopsis and cotton; how methods for manipulation of model systems used in the above work can be applied to expand our understanding of less studied surface structures of plants.

SCOPE

The Briefing will cover: established and suggested roles of simple and glandular secreting trichomes; recent results regarding solute and ion movement in trichomes; methods for isolating trichomes; recent studies of trichome differentiation and development; attempts to modify metabolism in secreting trichomes; efforts to exploit trichomes for commercial and agronomic purposes.

Recent developments in the use of transgenic plants for the production of human therapeutics and biopharmaceuticals

In recent years there has been a dramatic increase in the application of plant biotechnology for the production of a variety of commercially valuable simple and complex biological molecules (biologics) for use in human and animal healthcare. Transgenic whole plants and plant cell culture systems have been developed that have the capacity to economically produce large-scale quantities of antibodies and antibody fragments, antigens and/or vaccine epitopes, metabolic enzymes, hormones, (neuro)peptides and a variety of biologically active complexes and secondary metabolites for direct use as therapeutic agents or diagnostic tools in the medical healthcare industry. As the products of genetically modified plants make their way from concept to commercialization the associated risks and acceptance by the public has been become a focal point. In this paper, we summarize the recent advances made in the use of transgenic plants and plant cell cultures as biological factories for the production of human therapeutics and biopharmaceuticals and discuss the long-term potential of `molecular farming' as a low-cost, efficient method for the production of biological materials with demonstrated utility to the pharmaceutical industry or medical community.

A chloroplast transgenic approach to hyper-express and purify Human Serum Albumin, a protein highly susceptible to proteolytic degradation

Human Serum Albumin (HSA) accounts for 60% of the total protein in blood serum and it is the most widely used intravenous protein in a number of human therapies. HSA, however, is currently extracted only from blood because of a lack of commercially feasible recombinant expression systems. HSA is highly susceptible to proteolytic degradation in recombinant systems and is expensive to purify. Expression of HSA in transgenic chloroplasts using Shine-Dalgarno sequence (SD), which usually facilitates hyper-expression of transgenes, resulted only in 0.02% HSA in total protein (tp). Modification of HSA regulatory sequences using chloroplast untranslated regions (UTRs) resulted in hyper-expression of HSA (up to 11.1% tp), compensating for excessive proteolytic degradation. This is the highest expression of a pharmaceutical protein in transgenic plants and 500-fold greater than previous reports on HSA expression in transgenic leaves. Electron micrographs of immunogold labelled transgenic chloroplasts revealed HSA inclusion bodies, which provided a simple method for purification from other cellular proteins. HSA inclusion bodies could be readily solubilized to obtain a monomeric form using appropriate reagents. The regulatory elements used in this study should serve as a model system for enhancing expression of foreign proteins that are highly susceptible to proteolytic degradation and provide advantages in purification, when inclusion bodies are formed.

Targeting and expression of antigenic proteins in transgenic plants for production of edible oral vaccines

Exploiting plants as biological bioreactors for production and delivery of edible oral subunit vaccines is a promising application of biotechnology. Efforts to enhance expression levels of transgenes coding for antigenic proteins by exploiting promoters, targeting sequences, and enhancer elements have produced rather low quantities of the antigen in plant tissues, but enough to induce immune responses in feeding studies. This review will cover components of various gene constructs used in developing plant-based vaccines against a myriad of viral and bacterial diseases. Specifically, it will focus on sequences that are involved in targeting the antigen to mucosal tissues of the intestinal tract, thus enhancing the immunogenicity of the plant-based vaccine as well as those components that result in higher accumulation of the protein within the plant.