An in-built proteinase inhibitor system for the protection of recombinant proteins recovered from transgenic plants

Overexpression of RpKTI2 from Robinia pseudoacacia Affects the Photosynthetic Physiology and Endogenous Hormones of Tobacco

Kunitz trypsin inhibitor genes play important roles in stress resistance. In this study, we investigated RpKTI2 cloned from Robinia pseudoacacia and its effect on tobacco. RpKTI2 was introduced into the tobacco cultivar NC89 using Agrobacterium-mediated transformation. Six RpKTI2-overexpressing lines were obtained. Transgenic and wild-type tobacco plants were then compared for photosynthetic characteristics and endogenous hormone levels. Transgenic tobacco showed minor changes in chlorophyll content, fluorescence, and photosynthetic functions. However, the maximum photochemical efficiency (Fv/Fm) increased significantly while intercellular CO2 concentration (Ci) decreased significantly. Stomatal size and hormone content (indole-3-acetic acid, zeatin riboside, gibberellin, and indole-3-propionic acid) were reduced, while brassinosteroid content increased. Random forest regression revealed that RpKTI2 overexpression had the biggest impact on carotenoid content, initial fluorescence, Ci, stomatal area, and indole-3-acetic acid. Overall, RpKTI2 overexpression minimally affected chlorophyll synthesis and photosynthetic system characteristics but influenced stomatal development and likely enhanced the antioxidant capacity of tobacco. These findings provide a basis for future in-depth research on RpKTI2.

The advent of plant cells in bioreactors

Ever since agriculture started, plants have been bred to obtain better yields, better fruits, or sustainable products under uncertain biotic and abiotic conditions. However, a new way to obtain products from plant cells emerged with the development of recombinant DNA technologies. This led to the possibility of producing exogenous molecules in plants. Furthermore, plant chemodiversity has been the main source of pharmacological molecules, opening a field of plant biotechnology directed to produce high quality plant metabolites. The need for different products by the pharma, cosmetics agriculture and food industry has pushed again to develop new procedures. These include cell production in bioreactors. While plant tissue and cell culture are an established technology, beginning over a hundred years ago, plant cell cultures have shown little impact in biotechnology projects, compared to bacterial, yeasts or animal cells. In this review we address the different types of bioreactors that are currently used for plant cell production and their usage for quality biomolecule production. We make an overview of Nicotiana tabacum, Nicotiana benthamiana, Oryza sativa, Daucus carota, Vitis vinifera and Physcomitrium patens as well-established models for plant cell culture, and some species used to obtain important metabolites, with an insight into the type of bioreactor and production protocols.

Overexpression of soybean trypsin inhibitor genes decreases defoliation by corn earworm (Helicoverpa zea) in soybean (Glycine max) and Arabidopsis thaliana

Trypsin inhibitors (TIs) are widely distributed in plants and are known to play a protective role against herbivores. TIs reduce the biological activity of trypsin, an enzyme involved in the breakdown of many different proteins, by inhibiting the activation and catalytic reactions of proteins. Soybean (Glycine max) contains two major TI classes: Kunitz trypsin inhibitor (KTI) and Bowman-Birk inhibitor (BBI). Both genes encoding TI inactivate trypsin and chymotrypsin enzymes, which are the main digestive enzymes in the gut fluids of Lepidopteran larvae feeding on soybean. In this study, the possible role of soybean TIs in plant defense against insects and nematodes was investigated. A total of six TIs were tested, including three known soybean trypsin inhibitors (KTI1, KTI2 and KTI3) and three genes encoding novel inhibitors identified in soybean (KTI5, KTI7, and BBI5). Their functional role was further examined by overexpression of the individual TI genes in soybean and Arabidopsis. The endogenous expression patterns of these TI genes varied among soybean tissues, including leaf, stem, seed, and root. In vitro enzyme inhibitory assays showed significant increase in trypsin and chymotrypsin inhibitory activities in both transgenic soybean and Arabidopsis. Detached leaf-punch feeding bioassays detected significant reduction in corn earworm (Helicoverpa zea) larval weight when larvae fed on transgenic soybean and Arabidopsis lines, with the greatest reduction observed in KTI7 and BBI5 overexpressing lines. Whole soybean plant greenhouse feeding bioassays with H. zea on KTI7 and BBI5 overexpressing lines resulted in significantly reduced leaf defoliation compared to non-transgenic plants. However, bioassays of KTI7 and BBI5 overexpressing lines with soybean cyst nematode (SCN, Heterodera glycines) showed no differences in SCN female index between transgenic and non-transgenic control plants. There were no significant differences in growth and productivity between transgenic and non-transgenic plants grown in the absence of herbivores to full maturity under greenhouse conditions. The present study provides further insight into the potential applications of TI genes for insect resistance improvement in plants.

Transient proteolysis reduction of Nicotiana benthamiana-produced CAP256 broadly neutralizing antibodies using CRISPR/Cas9

The hypersensitive response is elicited by Agrobacterium infiltration of Nicotiana benthamiana, including the induction and accumulation of pathogenesis-related proteins, such as proteases. This includes the induction of the expression of several cysteine proteases from the C1 (papain-like cysteine protease) and C13 (legumain-like cysteine protease) families. This study demonstrates the role of cysteine proteases: NbVPE-1a, NbVPE-1b, and NbCysP6 in the proteolytic degradation of Nicotiana benthamiana (glycosylation mutant ΔXTFT)-produced anti-human immunodeficiency virus broadly neutralizing antibody, CAP256-VRC26.25. Three putative cysteine protease cleavage sites were identified in the fragment crystallizable region. We further demonstrate the transient coexpression of CAP256-VRC26.25 with CRISPR/Cas9-mediated genome editing vectors targeting the NbVPE-1a, NbVPE-1b, and NbCysP6 genes which resulted in a decrease in CAP256-VRC26.25 degradation. No differences in structural features were observed between the human embryonic kidney 293 (HEK293)-produced and ΔXTFT broadly neutralizing antibodies produced with and without the coexpression of genome-editing vectors. Furthermore, despite the presence of proteolytically degraded fragments of plant-produced CAP256-VRC26.25 without the coexpression of genome editing vectors, no influence on the in vitro functional activity was detected. Collectively, we demonstrate an innovative in planta strategy for improving the quality of the CAP256 antibodies through the transient expression of the CRISPR/Cas9 vectors.

CRISPR/Cas9-mediated knockout of the DCL2 and DCL4 genes in Nicotiana benthamiana and its productivity of recombinant proteins

KEY MESSAGE

DCL2 and DCL4 genes in Nicotiana benthamiana plants were successfully edited using the CRISPR/Cas9 system. Recently, plants have been utilized for recombinant protein production similar to other expression systems, i.e., bacteria, yeast, insect, and mammal cells. However, insufficient amounts of recombinant proteins are often produced in plant cells. The repression of RNA silencing within plant cells could improve production levels of recombinant protein because RNA silencing frequently decomposes mRNAs from transgenes. In this study, the genes dicer-like protein 2 and 4 (NbDCL2 and NbDCL4) were successfully edited to produce double-knockout transgenic Nicotiana benthamiana plants (dcl2dcl4 plants) using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology. A transient green fluorescent protein (GFP) gene expression assay revealed that the dcl2dcl4 plants accumulated higher amounts of GFP and GFP mRNA than wild type (WT) and RNA-dependent RNA polymerase 6-knockout N. benthamiana plants (ΔRDR6 plants). Small RNA sequencing also showed that dcl2dcl4 plants accumulated lower amounts of small interfering RNAs (siRNAs) against the GFP gene than WT plants. The dcl2dcl4 plants might also produce higher amounts of human fibroblast growth factor 1 (FGF1) than WT and ΔRDR6 plants. These observations appear to reflect differences between DCLs and RDR6 in plant cell biological mechanisms. These results reveal that dcl2dcl4 plants would be suitable as platform plants for recombinant protein production.

Engineering Approaches in Plant Molecular Farming for Global Health

Since the demonstration of the first plant-produced proteins of medical interest, there has been significant growth and interest in the field of plant molecular farming, with plants now being considered a viable production platform for vaccines. Despite this interest and development by a few biopharmaceutical companies, plant molecular farming is yet to be embraced by ‘big pharma’. The plant system offers a faster alternative, which is a potentially more cost-effective and scalable platform for the mass production of highly complex protein vaccines, owing to the high degree of similarity between the plant and mammalian secretory pathway. Here, we identify and address bottlenecks in the use of plants for vaccine manufacturing and discuss engineering approaches that demonstrate both the utility and versatility of the plant production system as a viable biomanufacturing platform for global health. Strategies for improving the yields and quality of plant-produced vaccines, as well as the incorporation of authentic posttranslational modifications that are essential to the functionality of these highly complex protein vaccines, will also be discussed. Case-by-case examples are considered for improving the production of functional protein-based vaccines. The combination of all these strategies provides a basis for the use of cutting-edge genome editing technology to create a general plant chassis with reduced host cell proteins, which is optimised for high-level protein production of vaccines with the correct posttranslational modifications.

CRISPR/Cas9-mediated knockout of the RDR6 gene in Nicotiana benthamiana for efficient transient expression of recombinant proteins

MAIN CONCLUSION

RDR6 gene knockout Nicotiana benthamiana plant was successfully produced using CRISPR/Cas9 technology. The production of recombinant proteins in plants has many advantages, such as safety and reduced costs. However, there are several problems with this technology, especially low levels of protein production. The dysfunction of the RNA silencing mechanism in plant cells would be effective to improve recombinant protein production because the RNA silencing mechanism efficiently degrades transgene-derived mRNAs. Therefore, to overcome this problem, clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology was used to develop RNA silencing-related gene knockout transgenic Nicotiana benthamiana. We successfully produced RNA-dependent RNA polymerase 6 (RDR6), one of the most important components of the RNA silencing mechanism-knockout N. benthamiana (ΔRDR6 plants). The ΔRDR6 plants had abnormal flowers and were sterile, as with the Arabidopsis RDR6 mutants. However, a transient gene expression assay showed that the ΔRDR6 plants accumulated larger amounts of green fluorescent protein (GFP) and GFP mRNA than the wild-type (WT) plants. Small RNA sequencing analysis revealed that levels of small interfering RNA against the GFP gene were greatly reduced in the ΔRDR6 plants, as compared to that of the WT plants. These findings demonstrate that the ΔRDR6 plants can express larger amounts of recombinant proteins than WT plants and, therefore, would be useful for recombinant protein production and understanding the contributions of RDR6 to genetic and physiological events in plants.

Recombinant cystatins in plants

Dozens of studies have assessed the practical value of plant cystatins as ectopic inhibitors of Cys proteases in biological systems. The potential of these proteins in crop protection to control herbivorous pests and pathogens has been documented extensively over the past 25 years. Their usefulness to regulate endogenous Cys proteases in planta has also been considered recently, notably to implement novel traits of agronomic relevance in crops or to generate protease activity-depleted environments in plants or plant cells used as bioreactors for recombinant proteins. After a brief update on the basic structural characteristics of plant cystatins, we summarize recent advances on the use of these proteins in plant biotechnology. Attention is also paid to the molecular improvement of their structural properties for the improvement of their protease inhibitory effects or the fine-tuning of their biological target range.

Plant Serine Protease Inhibitors: Biotechnology Application in Agriculture and Molecular Farming

The serine protease inhibitors (SPIs) are widely distributed in living organisms like bacteria, fungi, plants, and humans. The main function of SPIs as protease enzymes is to regulate the proteolytic activity. In plants, most of the studies of SPIs have been focused on their physiological role. The initial studies carried out in plants showed that SPIs participate in the regulation of endogenous proteolytic processes, as the regulation of proteases in seeds. Besides, it was observed that SPIs also participate in the regulation of cell death during plant development and senescence. On the other hand, plant SPIs have an important role in plant defense against pests and phytopathogenic microorganisms. In the last 20 years, several transgenic plants over-expressing SPIs have been produced and tested in order to achieve the increase of the resistance against pathogenic insects. Finally, in molecular farming, SPIs have been employed to minimize the proteolysis of recombinant proteins expressed in plants. The present review discusses the potential biotechnological applications of plant SPIs in the agriculture field.

Low Protease Content in Medicago truncatula Cell Cultures Facilitates Recombinant Protein Production

Medicago truncatula is an established model for studying legume biology. More recently, it has also been exploited as a Molecular Farming platform for the production of recombinant proteins, with the successful expression of fungal and human proteins in plants and cell suspension cultures of this species. One of the challenges that now must be overcome is the degradation of final products during production and downstream processing stages. In the M. truncatula genome, there are more than 400 putative protease-encoding genes, but to date, the proteolytic content of Medicago cell cultures has not been studied. In this report, the proteolytic activities that can potentially hamper the successful production of recombinant proteins in this system are evaluated. The potential proteases responsible for the degradation of target proteins are identified. Interestingly, the number of proteases found in Medicago spent medium is considerably lower than that of the well-established tobacco bright yellow 2 (BY-2) system. Papain-like cysteine proteases are found to be the major contributors to recombinant protein degradation in Medicago. This knowledge is used to engineer a cell line with reduced endogenous protease activity by expressing a selective protease inhibitor, further improving this expression platform.

Repression of the DCL2 and DCL4 genes in Nicotiana benthamiana plants for the transient expression of recombinant proteins

The production of recombinant proteins in plants has many advantages, including safety and reduced costs. However, this technology still faces several issues, including low levels of production. The repression of RNA silencing seems to be particularly important for improving recombinant protein production because RNA silencing effectively degrades transgene-derived mRNAs in plant cells. Therefore, to overcome this, we used RNA interference technology to develop DCL2- and DCL4-repressed transgenic Nicotiana benthamiana plants (ΔD2, ΔD4, and ΔD2ΔD4 plants), which had much lower levels of NbDCL2 and/or NbDCL4 mRNAs than wild-type plants. A transient gene expression assay showed that the ΔD2ΔD4 plants accumulated larger amounts of green fluorescent protein (GFP) and human acidic fibroblast growth factor (aFGF) than ΔD2, ΔD4, and wild-type plants. Furthermore, the levels of GFP and aFGF mRNAs were also higher in ΔD2ΔD4 plants than in ΔD2, ΔD4, and wild-type plants. These findings demonstrate that ΔD2ΔD4 plants express larger amounts of recombinant proteins than wild-type plants, and so would be useful for recombinant protein production.

Functional proteomics-aided selection of protease inhibitors for herbivore insect control

Studies have reported the potential of protease inhibitors to engineer insect resistance in transgenic plants but the general usefulness of this approach in crop protection still remains to be established. Insects have evolved strategies to cope with dietary protease inhibitors, such as the use of proteases recalcitrant to inhibition, that often make the selection of effective inhibitors very challenging. Here, we used a functional proteomics approach for the ‘capture’ of Cys protease targets in crude protein extracts as a tool to identify promising cystatins for plant improvement. Two cystatins found to differ in their efficiency to capture Cys proteases of the coleopteran pest Leptinotarsa decemlineata also differed in their usefulness to produce transgenic potato lines resistant to this insect. Plants expressing the most potent cystatin at high level had a strong repressing effect on larval growth and leaf intake, while plants expressing the weakest cystatin showed no effect on both two parameters compared to untransformed parental line used for genetic transformation. Our data underline the relevance of considering the whole range of possible protease targets when selecting an inhibitor for plant pest control. They also confirm the feasibility of developing cystatin-expressing transgenics resistant to a major pest of potato.

An Accessory Protease Inhibitor to Increase the Yield and Quality of a Tumour-Targeting mAb in Nicotiana benthamiana Leaves

The overall quality of recombinant IgG antibodies in plants is dramatically compromised by host endogenous proteases. Different approaches have been developed to reduce the impact of endogenous proteolysis on IgGs, notably involving site-directed mutagenesis to eliminate protease-susceptible sites or the in situ mitigation of host protease activities to minimize antibody processing in the cell secretory pathway. We here characterized the degradation profile of H10, a human tumour-targeting monoclonal IgG, in leaves of Nicotiana benthamiana also expressing the human serine protease inhibitor α1-antichymotrypsin or the cysteine protease inhibitor tomato cystatin SlCYS8. Leaf extracts revealed consistent fragmentation patterns for the recombinant antibody regardless of leaf age and a strong protective effect of SlCYS8 in specific regions of the heavy chain domains. As shown using an antigen-binding ELISA and LC-MS/MS analysis of antibody fragments, SlCYS8 had positive effects on both the amount of fully-assembled antibody purified from leaf tissue and the stability of biologically active antibody fragments containing the heavy chain Fc domain. Our data confirm the potential of Cys protease inhibitors as convenient antibody-stabilizing expression partners to increase the quality of therapeutic antibodies in plant protein biofactories.

Site-targeted mutagenesis for stabilization of recombinant monoclonal antibody expressed in tobacco (Nicotiana tabacum) plants

This study examined the degradation pattern of a murine IgG1κ monoclonal antibody expressed in and extracted from transformedNicotiana tabacum Gel electrophoresis of leaf extracts revealed a consistent pattern of recombinant immunoglobulin bands, including intact and full-length antibody, as well as smaller antibody fragments. N-terminal sequencing revealed these smaller fragments to be proteolytic cleavage products and identified a limited number of protease-sensitive sites in the antibody light and heavy chain sequences. No strictly conserved target sequence was evident, although the peptide bonds that were susceptible to proteolysis were predominantly and consistently located within or near to the interdomain or solvent-exposed regions in the antibody structure. Amino acids surrounding identified cleavage sites were mutated in an attempt to increase resistance. Different Guy's 13 antibody heavy and light chain mutant combinations were expressed transiently inN. tabacumand demonstrated intensity shifts in the fragmentation pattern, resulting in alterations to the full-length antibody-to-fragment ratio. The work strengthens the understanding of proteolytic cleavage of antibodies expressed in plants and presents a novel approach to stabilize full-length antibody by site-directed mutagenesis.-Hehle, V. K., Paul, M. J., Roberts, V. A., van Dolleweerd, C. J., Ma, J. K.-C. Site-targeted mutagenesis for stabilization of recombinant monoclonal antibody expressed in tobacco (Nicotiana tabacum) plants.

Companion Protease Inhibitors for the In Situ Protection of Recombinant Proteins in Plants

We previously described a procedure for the use of plant protease inhibitors as "companion" accessory proteins to prevent unwanted proteolysis of clinically useful recombinant proteins in leaf crude protein extracts (Benchabane et al. Methods Mol Biol 483:265-273, 2009). Here we describe the use of these inhibitors for the protection of recombinant proteins in planta, before their extraction from leaf tissues. A procedure is first described involving inhibitors co-expressed along-and co-migrating-with the protein of interest in host plant cells. An alternative, single transgene scheme is then described involving translational fusions of the recombinant protein and companion inhibitor. These approaches may allow for a significant improvement of protein steady-state levels in leaves, comparable to yield improvements observed with protease-deficient strains of less complex protein expression hosts such as E. coli or yeasts.

Leaf proteome rebalancing in Nicotiana benthamiana for upstream enrichment of a transiently expressed recombinant protein

A key factor influencing the yield of biopharmaceuticals in plants is the ratio of recombinant to host proteins in crude extracts. Postextraction procedures have been devised to enrich recombinant proteins before purification. Here, we assessed the potential of methyl jasmonate (MeJA) as a generic trigger of recombinant protein enrichment in Nicotiana benthamiana leaves before harvesting. Previous studies have reported a significant rebalancing of the leaf proteome via the jasmonate signalling pathway, associated with ribulose 1,5-bisphosphate carboxylase oxygenase (RuBisCO) depletion and the up-regulation of stress-related proteins. As expected, leaf proteome alterations were observed 7 days post-MeJA treatment, associated with lowered RuBisCO pools and the induction of stress-inducible proteins such as protease inhibitors, thionins and chitinases. Leaf infiltration with the Agrobacterium tumefaciens bacterial vector 24 h post-MeJA treatment induced a strong accumulation of pathogenesis-related proteins after 6 days, along with a near-complete reversal of MeJA-mediated stress protein up-regulation. RuBisCO pools were partly restored upon infiltration, but most of the depletion effect observed in noninfiltrated plants was maintained over six more days, to give crude protein samples with 50% less RuBisCO than untreated tissue. These changes were associated with net levels reaching 425 μg/g leaf tissue for the blood-typing monoclonal antibody C5-1 expressed in MeJA-treated leaves, compared to less than 200 μg/g in untreated leaves. Our data confirm overall the ability of MeJA to trigger RuBisCO depletion and recombinant protein enrichment in N. benthamiana leaves, estimated here for C5-1 at more than 2-fold relative to host proteins.

Extracellular peptidase hunting for improvement of protein production in plant cells and roots

Plant-based recombinant protein production systems have gained an extensive interest over the past few years, because of their reduced cost and relative safety. Although the first products are now reaching the market, progress are still needed to improve plant hosts and strategies for biopharming. Targeting recombinant proteins toward the extracellular space offers several advantages in terms of protein folding and purification, but degradation events are observed, due to endogenous peptidases. This paper focuses on the analysis of extracellular proteolytic activities in two production systems: cell cultures and root-secretion (rhizosecretion), in Arabidopsis thaliana and Nicotiana tabacum. Proteolytic activities of extracellular proteomes (secretomes) were evaluated in vitro against two substrate proteins: bovine serum albumin (BSA) and human serum immunoglobulins G (hIgGs). Both targets were found to be degraded by the secretomes, BSA being more prone to proteolysis than hIgGs. The analysis of the proteolysis pH-dependence showed that target degradation was mainly dependent upon the production system: rhizosecretomes contained more peptidase activity than extracellular medium of cell suspensions, whereas variations due to plant species were smaller. Using class-specific peptidase inhibitors, serine, and metallopeptidases were found to be responsible for degradation of both substrates. An in-depth in silico analysis of genomic and transcriptomic data from Arabidopsis was then performed and led to the identification of a limited number of serine and metallo-peptidases that are consistently expressed in both production systems. These peptidases should be prime candidates for further improvement of plant hosts by targeted silencing.

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.

Ectopic phytocystatin expression leads to enhanced drought stress tolerance in soybean (Glycine max) and Arabidopsis thaliana through effects on strigolactone pathways and can also result in improved seed traits

Ectopic cystatin expression has long been used in plant pest management, but the cysteine protease, targets of these inhibitors, might also have important functions in the control of plant lifespan and stress tolerance that remain poorly characterized. We therefore characterized the effects of expression of the rice cystatin, oryzacystatin-I (OCI), on the growth, development and stress tolerance of crop (soybean) and model (Arabidopsis thaliana) plants. Ectopic OCI expression in soybean enhanced shoot branching and leaf chlorophyll accumulation at later stages of vegetative development and enhanced seed protein contents and decreased the abundance of mRNAs encoding strigolactone synthesis enzymes. The OCI-expressing A. thaliana showed a slow-growth phenotype, with increased leaf numbers and enhanced shoot branching at flowering. The OCI-dependent inhibition of cysteine proteases enhanced drought tolerance in soybean and A. thaliana, photosynthetic CO2 assimilation being much less sensitive to drought-induced inhibition in the OCI-expressing soybean lines. Ectopic OCI expression or treatment with the cysteine protease inhibitor E64 increased lateral root densities in A. thaliana. E64 treatment also increased lateral root densities in the max2-1 mutants that are defective in strigolactone signalling, but not in the max3-9 mutants that are defective in strigolactone synthesis. Taken together, these data provide evidence that OCI-inhibited cysteine proteases participate in the control of growth and stress tolerance through effects on strigolactones. We conclude that cysteine proteases are important targets for manipulation of plant growth, development and stress tolerance, and also seed quality traits.

Inhibition of protease activity by antisense RNA improves recombinant protein production in Nicotiana tabacum cv. Bright Yellow 2 (BY-2) suspension cells

Recombinant proteins produced in plant suspension cultures are often degraded by endogenous plant proteases when secreted into the medium, resulting in low yields. To generate protease-deficient tobacco BY-2 cell lines and to retrieve the sequence information, we cloned four different protease cDNAs from tobacco BY-2 cells (NtAP, NtCP, NtMMP1, and NtSP), which represent the major catalytic classes. The simultaneous expression of antisense RNAs against these endogenous proteases led to the establishment of cell lines with reduced levels of endogenous protease expression and activity at late stages of the cultivation cycle. One of the cell lines showing reduced proteolytic activity in the culture medium was selected for the expression of the recombinant full-length IgG1(κ) antibody 2F5, recognizing the gp41 surface protein of HIV-1. This cell line showed significantly reduced degradation of the 2F5 heavy chain, resulting in four-fold higher accumulation of the intact antibody heavy chain when compared to transformed wild type cells expressing the same antibody. N-terminal sequencing data revealed that the antibody has two cleavage sites within the CDR-H3 and one site at the end of the H4-framework region. These cleavage sites are found to be vulnerable to serine proteases. The data provide a basis for further improvement of plant cells for the production of recombinant proteins in plant cell suspension cultures.

The human anti-HIV antibodies 2F5, 2G12, and PG9 differ in their susceptibility to proteolytic degradation: down-regulation of endogenous serine and cysteine proteinase activities could improve antibody production in plant-based expression platforms

The tobacco-related species Nicotiana benthamiana has recently emerged as a promising host for the manufacturing of protein therapeutics. However, the production of recombinant proteins in N. benthamiana is frequently hampered by undesired proteolysis. Here, we show that the expression of the human anti-HIV antibodies 2F5, 2G12, and PG9 in N. benthamiana leaves leads to the accumulation of discrete heavy chain-derived degradation products of 30-40 kDa. Incubation of purified 2F5 with N. benthamiana intercellular fluid resulted in rapid conversion into the 40-kDa fragment, whereas 2G12 proved largely resistant to degradation. Such a differential susceptibility to proteolytic attack was also observed when these two antibodies were exposed to various types of proteinases in vitro. While serine and cysteine proteinases are both capable of generating the 40-kDa 2F5 fragment, the 30-kDa polypeptide is most readily obtained by treatment with the latter class of enzymes. The principal cleavage sites reside within the antigen-binding domain, the VH -CH 1 linker segment and the hinge region of the antibodies. Collectively, these results indicate that down-regulation of endogenous serine and cysteine proteinase activities could be used to improve the performance of plant-based expression platforms destined for the production of biopharmaceuticals.

Proteolysis of recombinant proteins in bioengineered plant cells

Plants are increasingly used as alternative expression hosts for the production of recombinant proteins offering many advantages including higher biomass and the ability to perform post-translational modifications on complex proteins. Key challenges for optimized accumulation of recombinant proteins in a plant system still remain, including endogenous plant proteolytic activity, which may severely compromise recombinant protein stability. Several strategies have recently been applied to improve protein stability by limiting protease action such as recombinant protein production in various sub-cellular compartments or application of protease inhibitors to limit protease action. A short update on the current strategies applied is provided here, with particular focus on sub-cellular sites previously selected for recombinant protein production and the co-expression of protease inhibitors to limit protease activity.

Tomato cystatin SlCYS8 as a stabilizing fusion partner for human serpin expression in plants

Studies have reported the usefulness of fusion proteins to bolster recombinant protein yields in plants. Here, we assess the potential of tomato SlCYS8, a Cys protease inhibitor of the cystatin protein superfamily, as a stabilizing fusion partner for human alpha-1-antichymotrypsin (α1ACT) targeted to the plant cell secretory pathway. Using the model expression platform Nicotiana benthamiana, we show that the cystatin imparts a strong stabilizing effect when expressed as a translational fusion with α1ACT, allowing impressive accumulation yields of over 2 mg/g of fresh weight tissue for the human serpin, a 25-fold improvement on the yield of α1ACT expressed alone. Natural and synthetic peptide linkers inserted between SlCYS8 and α1ACT have differential effects on protease inhibitory potency of the two protein partners in vitro. They also have a differential impact on the yield of α1ACT, dependent on the extent to which the hybrid protein may remain intact in the plant cell environment. The stabilizing effect of SlCYS8 does not involve Cys protease inhibition and can be partly reproduced in the cytosol, where peptide linkers are less susceptible to degradation. The effect of SlCYS8 on α1ACT yields could be explained by: (i) an improved translation of the human protein coding sequence; and/or (ii) an overall stabilization of its tertiary structure preventing proteolytic degradation and/or polymerization. These findings suggest the potential of plant cystatins as stabilizing fusion partners for recombinant proteins in plant systems. They also underline the need for an empirical assessment of peptide linker functions in plant cell environments.

Protection of recombinant mammalian antibodies from development-dependent proteolysis in leaves of Nicotiana benthamiana

The expression of clinically useful proteins in plants has been bolstered by the development of high-yielding systems for transient protein expression using agroinfiltration. There is a need now to know more about how host plant development and metabolism influence the quantity and quality of recombinant proteins. Endogenous proteolysis is a key determinant of the stability and yield of recombinant proteins in plants. Here we characterised cysteine (C1A) and aspartate (A1) protease profiles in leaves of the widely used expression host Nicotiana benthamiana, in relation with the production of a murine IgG, C5-1, targeted to the cell secretory pathway. Agroinfiltration significantly altered the distribution of C1A and A1 proteases along the leaf age gradient, with a correlation between leaf age and the level of proteolysis in whole-cell and apoplast protein extracts. The co-expression of tomato cystatin SlCYS8, an inhibitor of C1A proteases, alongside C5-1 increased antibody yield by nearly 40% after the usual 6-days incubation period, up to ~3 mg per plant. No positive effect of SlCYS8 was observed in oldest leaves, in line with an increased level of C1A protease activity and a very low expression rate of the inhibitor. By contrast, C5-1 yield was greater by an additional 40% following 8- to 10-days incubations in younger leaves, where high SlCYS8 expression was maintained. These findings confirm that the co-expression of recombinant protease inhibitors is a promising strategy for increasing recombinant protein yields in plants, but that further opportunity exists to improve this approach by addressing the influence of leaf age and proteases of other classes.

Wounding, insect chewing and phloem sap feeding differentially alter the leaf proteome of potato, Solanum tuberosum L

Background

Various factors shape the response of plants to herbivorous insects, including wounding patterns, specific chemical effectors and feeding habits of the attacking herbivore. Here we performed a comparative proteomic analysis of the plant's response to wounding and herbivory, using as a model potato plants (Solanum tuberosum L.) subjected to mechanical wounding, defoliation by the Colorado potato beetle Leptinotarsa decemlineata Say, or phloem sap feeding by the potato aphid Macrosiphum euphorbiae Thomas.

Results

Out of ~500 leaf proteins monitored by two-dimensional gel electrophoresis (2-DE), 31 were up- or downregulated by at least one stress treatment compared to healthy control plants. Of these proteins, 29 were regulated by beetle chewing, 8 by wounding and 8 by aphid feeding. Some proteins were up- or downregulated by two different treatments, while others showed diverging expression patterns in response to different treatments. A number of modulated proteins identified by mass spectrometry were typical defense proteins, including wound-inducible protease inhibitors and pathogenesis-related proteins. Proteins involved in photosynthesis were also modulated, notably by potato beetle feeding inducing a strong decrease of some photosystem I proteins. Quantitative RT PCR assays were performed with nucleotide primers for photosynthesis-related proteins to assess the impact of wounding and herbivory at the gene level. Whereas different, sometimes divergent, responses were observed at the proteome level in response to wounding and potato beetle feeding, downregulating effects were systematically observed for both treatments at the transcriptional level.

Conclusions

These observations illustrate the differential impacts of wounding and insect herbivory on defense- and photosynthesis-related components of the potato leaf proteome, likely associated with the perception of distinct physical and chemical cues in planta.

High-level transient expression of ER-targeted human interleukin 6 in Nicotiana benthamiana

Tobacco plants can be used to express recombinant proteins that cannot be produced in a soluble and active form using traditional platforms such as Escherichia coli. We therefore expressed the human glycoprotein interleukin 6 (IL6) in two commercial tobacco cultivars (Nicotiana tabacum cv. Virginia and cv. Geudertheimer) as well as the model host N. benthamiana to compare different transformation strategies (stable vs. transient expression) and subcellular targeting (apoplast, endoplasmic reticulum (ER) and vacuole). In T(0) transgenic plants, the highest expression levels were achieved by ER targeting but the overall yields of IL6 were still low in the leaves (0.005% TSP in the ER, 0.0008% in the vacuole and 0.0005% in the apoplast). The apoplast variant accumulated to similar levels in leaves and seeds, whereas the ER-targeted variant was 1.2-fold more abundant in seeds and the vacuolar variant was 6-fold more abundant in seeds. The yields improved in subsequent generations, with the best-performing T(2) plants producing the ER-targeted IL6 at 0.14% TSP in both leaves and seeds. Transient expression of ER-targeted IL6 in leaves using the MagnICON system resulted in yields of up to 7% TSP in N. benthamiana, but only 1% in N. tabacum cv. Virginia and 0.5% in cv. Geudertheimer. Although the commercial tobacco cultivars produced up to threefold more biomass than N. benthamiana, this was not enough to compensate for the lower overall yields. The recombinant IL6 produced by transient and stable expression in plants was biologically active and presented as two alternative bands matching the corresponding native protein.

Beneficial 'unintended effects' of a cereal cystatin in transgenic lines of potato, Solanum tuberosum

BACKGROUND

Studies reported unintended pleiotropic effects for a number of pesticidal proteins ectopically expressed in transgenic crops, but the nature and significance of such effects in planta remain poorly understood. Here we assessed the effects of corn cystatin II (CCII), a potent inhibitor of C1A cysteine (Cys) proteases considered for insect and pathogen control, on the leaf proteome and pathogen resistance status of potato lines constitutively expressing this protein.

RESULTS

The leaf proteome of lines accumulating CCII at different levels was resolved by 2-dimensional gel electrophoresis and compared with the leaf proteome of a control (parental) line. Out of ca. 700 proteins monitored on 2-D gels, 23 were significantly up- or downregulated in CCII-expressing leaves, including 14 proteins detected de novo or up-regulated by more than five-fold compared to the control. Most up-regulated proteins were abiotic or biotic stress-responsive proteins, including different secretory peroxidases, wound inducible protease inhibitors and pathogenesis-related proteins. Accordingly, infection of leaf tissues by the fungal necrotroph Botryris cinerea was prevented in CCII-expressing plants, despite a null impact of CCII on growth of this pathogen and the absence of extracellular Cys protease targets for the inhibitor.

CONCLUSIONS

These data point to the onset of pleiotropic effects altering the leaf proteome in transgenic plants expressing recombinant protease inhibitors. They also show the potential of these proteins as ectopic modulators of stress responses in planta, useful to engineer biotic or abiotic stress tolerance in crop plants of economic significance.

Use of transgenic oryzacystatin-I-expressing plants enhances recombinant protein production

Plants are an effective and inexpensive host for the production of commercially interesting heterologous recombinant proteins. The Escherichia coli-derived glutathione reductase was transiently expressed as a recombinant model protein in the cytosol of tobacco plants using the technique of leaf agro-infiltration. Proteolytic cysteine protease activity progressively increased over time when glutathione reductase accumulated in leaves. Application of cysteine protease promoter-GUS fusions in transgenic tobacco identified a cysteine protease NtCP2 expressed in mature leaves and being stress responsive to be expressed as a consequence of agro-infiltration. Transgenic tobacco plants constitutively expressing the rice cysteine protease inhibitor oryzacystatin-I had significantly lower cysteine protease activity when compared to non-transgenic tobacco plants. Lower cysteine protease activity in transgenic plants was directly related to higher glutathione reductase activity and also higher glutathione reductase amounts in transgenic plants. Overall, our work has demonstrated as a novel aspect that transgenic tobacco plants constitutively expressing an exogenous cysteine protease inhibitor have the potential for producing more recombinant protein which is very likely due to the reduced activity of endogenous cysteine protease.

Microbial and fungal protease inhibitors--current and potential applications

Proteolytic enzymes play essential metabolic and regulatory functions in many biological processes and also offer a wide range of biotechnological applications. Because of their essential roles, their proteolytic activity needs to be tightly regulated. Therefore, small molecules and proteins that inhibit proteases can be versatile tools in the fields of medicine, agriculture and biotechnology. In medicine, protease inhibitors can be used as diagnostic or therapeutic agents for viral, bacterial, fungal and parasitic diseases as well as for treating cancer and immunological, neurodegenerative and cardiovascular diseases. They can be involved in crop protection against plant pathogens and herbivorous pests as well as against abiotic stress such as drought. Furthermore, protease inhibitors are indispensable in protein purification procedures to prevent undesired proteolysis during heterologous expression or protein extraction. They are also valuable tools for simple and effective purification of proteases, using affinity chromatography. Because there are such a large number and diversity of proteases in prokaryotes, yeasts, filamentous fungi and mushrooms, we can expect them to be a rich source of protease inhibitors as well.

A protease activity-depleted environment for heterologous proteins migrating towards the leaf cell apoplast

Recombinant proteins face major constraints along the plant cell secretory pathway, including proteolytic processing compromising their structural integrity. Here, we demonstrate the potential of protease inhibitors as in situ stabilizing agents for recombinant proteins migrating towards the leaf apoplast. Genomic data for Arabidopsis, rice and Nicotiana spp. were assessed to determine the relative incidence of protease families in the cell secretory pathway. Transient expression assays with the model platform Nicotiana benthamiana were then performed to test the efficiency of protease inhibitors in stabilizing proteins targeted to the apoplast. Current genomic data suggest the occurrence of proteases from several families along the secretory pathway, including A1 and A22 Asp proteases; C1A and C13 Cys proteases; and S1, S8 and S10 Ser proteases. In vitro protease assays confirmed the presence of various proteases in N. benthamiana leaves, notably pointing to the deposition of A1- and S1-type activities preferentially in the apoplast. Accordingly, transient expression and secretion of the A1/S1 protease inhibitor, tomato cathepsin D inhibitor (SlCDI), negatively altered A1 and S1 protease activities in this cell compartment, while increasing the leaf apoplast protein content by ∼45% and improving the accumulation of a murine diagnostic antibody, C5-1, co-secreted in the apoplast. SlCYS9, an inhibitor of C1A and C13 Cys proteases, had no impact on the apoplast proteases and protein content, but stabilized C5-1 in planta, presumably upstream in the secretory pathway. These data confirm, overall, the potential of protease inhibitors for the in situ protection of recombinant proteins along the plant cell secretory pathway.

Antibody degradation in tobacco plants: a predominantly apoplastic process

BACKGROUND

Interest in using plants for production of recombinant proteins such as monoclonal antibodies is growing, but proteolytic degradation, leading to a loss of functionality and complications in downstream purification, is still a serious problem.

RESULTS

In this study, we investigated the dynamics of the assembly and breakdown of a human IgG(1)κ antibody expressed in plants. Initial studies in a human IgG transgenic plant line suggested that IgG fragments were present prior to extraction. Indeed, when the proteolytic activity of non-transgenic Nicotiana tabacum leaf extracts was tested against a human IgG1 substrate, little activity was detectable in extraction buffers with pH > 5. Significant degradation was only observed when the plant extract was buffered below pH 5, but this proteolysis could be abrogated by addition of protease inhibitors. Pulse-chase analysis of IgG MAb transgenic plants also demonstrated that IgG assembly intermediates are present intracellularly and are not secreted, and indicates that the majority of proteolytic degradation occurs following secretion into the apoplastic space.

CONCLUSIONS

The results provide evidence that proteolytic fragments derived from antibodies of the IgG subtype expressed in tobacco plants do not accumulate within the cell, and are instead likely to occur in the apoplastic space. Furthermore, any proteolytic activity due to the release of proteases from subcellular compartments during tissue disruption and extraction is not a major consideration under most commonly used extraction conditions.

Plant bioreactors for pharmaceuticals

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

Recombinant protein expression in Nicotiana

Recombinant protein pharmaceuticals are now widely used in treatment of chronic diseases, and several recombinant protein subunit vaccines are approved for human and veterinary use. With growing demand for complex protein pharmaceuticals, such as monoclonal antibodies, manufacturing capacity is becoming limited. There is increasing need for safe, scalable, and economical alternatives to mammalian cell culture-based manufacturing systems, which require substantial capital investment for new manufacturing facilities. Since a seminal paper reporting immunoglobulin expression in transgenic plants was published in 1989, there have been many technological advances in plant expression systems to the present time where production of proteins in leaf tissues of nonfood crops such as Nicotiana species is considered a viable alternative. In particular, transient expression systems derived from recombinant plant viral vectors offer opportunities for rapid expression screening, construct optimization, and expression scale-up. Extraction of recombinant proteins from Nicotiana leaf tissues can be achieved by collection of secreted protein fractions, or from a total protein extract after grinding the leaves with buffer. After separation from solids, the major purification challenge is contamination with elements of the photosynthetic complex, which can be solved by application of a variety of facile and proven strategies. In conclusion, the technologies required for safe, efficient, scalable manufacture of recombinant proteins in Nicotiana leaf tissues have matured to the point where several products have already been tested in phase I clinical trials and will soon be followed by a rich pipeline of recombinant vaccines, microbicides, and therapeutic proteins.

Recombinant protease inhibitors for herbivore pest control: a multitrophic perspective

Protease inhibitors are a promising complement to Bt toxins for the development of insect-resistant transgenic crops, but their limited specificity against proteolytic enzymes and the ubiquity of protease-dependent processes in living organisms raise questions about their eventual non-target effects in agroecosystems. After a brief overview of the main factors driving the impacts of insect-resistant transgenic crops on non-target organisms, the possible effects of protease inhibitors are discussed from a multitrophic perspective, taking into account not only the target herbivore proteases but also the proteases of other organisms found along the trophic chain, including the plant itself. Major progress has been achieved in recent years towards the design of highly potent broad-spectrum inhibitors and the field deployment of protease inhibitor-expressing transgenic plants resistant to major herbivore pests. A thorough assessment of the current literature suggests that, whereas the non-specific inhibitory effects of recombinant protease inhibitors in plant food webs could often be negligible and their 'unintended' pleiotropic effects in planta of potential agronomic value, the innocuity of these proteins might always remain an issue to be assessed empirically, on a case-by-case basis.

2-DE proteome maps for the leaf apoplast of Nicotiana benthamiana

We provide 2-D gel reference maps for the apoplastic proteome of Nicotiana benthamiana leaves infiltrated or not with the bacterial gene vector Agrobacterium tumefaciens. About 90 proteins were analyzed by LC-MS/MS for identification and function assignment. We show, overall, an effective response of the plant to agroinfiltration involving a specific, cell wall maintenance-independent up-regulation of defense protein secretion. The proteome maps described should be a useful tool for systemic studies on plant-pathogen interactions or cell wall metabolism. They also should prove useful for the monitoring of secreted recombinant proteins and their possible pleiotropic effects along the cell secretory pathway of N. benthamiana leaves used as an expression platform for clinically useful proteins.

Plant cystatins

Plant cystatins have been the object of intense research since the publication of a first paper reporting their existence more than 20 years ago. These ubiquitous inhibitors of Cys proteases play several important roles in plants, from the control of various physiological and cellular processes in planta to the inhibition of exogenous Cys proteases secreted by herbivorous arthropods and pathogens to digest or colonize plant tissues. After an overview of current knowledge about the evolution, structure and inhibitory mechanism of plant cystatins, we review the different roles attributed to these proteins in plants. The potential of recombinant plant cystatins as effective pesticidal proteins in crop protection is also considered, as well as protein engineering approaches adopted over the years to improve their inhibitory potency and specificity towards Cys proteases of biotechnological interest.

Report of the fourth meeting on 'Influenza vaccines that induce broad spectrum and long-lasting immune responses', World Health Organization and Wellcome Trust, London, United Kingdom, 9-10 November 2009

Current influenza vaccines are limited by the need for annual immunisation, frequent antigenic updating to match the evolution of circulating influenza virus strains, and reduced efficacy in elderly persons. On 9-10 November 2009, the Initiative for Vaccine Research of the World Health Organization convened jointly with the Wellcome Trust in London, United Kingdom, the fourth meeting on 'Influenza vaccines that induce broad spectrum and long-lasting immune responses'. Presentations were made by representatives from industry, academia, governmental and non-governmental organisations. The objectives of the meeting were to update the progress of research in the field of influenza vaccine strategies able to generate cross protection against divergent influenza virus strains. Improvements in existing strategies including live attenuated influenza vaccines and adjuvantation of inactivated vaccines were summarised. Developments in novel antigen production methods, new routes of vaccine delivery and administration, and vaccine approaches based on conserved virus antigens were explored. In addition, correlates of immune protection and regulatory issues for the evaluation and approval of future novel vaccine strategies were discussed.

A companion protease inhibitor for the protection of cytosol-targeted recombinant proteins in plants

We reported earlier the potential of tomato cathepsin D inhibitor (SlCDI) as an in-built stabilizing agent for the protection of recombinant proteins in transgenic plant leaf crude extracts (Plant Biotechnol J.4, 359-368). Here we document the potential of SlCDI for the in situ protection of proteins in potato leaves. Total protein assays with control and SlCDI-expressing potato lines indicated a positive impact of slcdi transgene expression on leaf protein content, with a mean relative increase of 35%-40% depending on the light regime. Out of approximately 700 proteins detected on 2-D gels, only 20 exhibited a significantly altered level on a protein-specific basis, whereas most proteins were up-regulated on a leaf fresh weight basis, albeit at variable rates. Quantitative reverse trancriptase-PCR assays for rubisco activase showed similar transcript levels in leaves of test and control lines despite protein levels increased by two- to threefold in SlCDI-expressing lines. These observations, along with the unrelated biological functions assigned to MS-identified proteins up-regulated in leaves and protease assays showing slightly increased proteasome activity in protein extracts of SlCDI-expressing lines, suggest a general, proteasome-independent protein stabilizing effect of SlCDI in planta. Transient expression assays with human alpha(1)-antichymotrypsin also showed a stabilizing effect for SlCDI on heterologous proteins, leading to net levels of the human protein increased by approximately 2.5-fold in SlCDI-expressing plants. These data illustrate, overall, the potential of SlCDI as an in vivo protein-stabilizing agent in transgenic plant systems, useful to improve protein levels and recombinant protein accumulation.

Tubers from potato lines expressing a tomato Kunitz protease inhibitor are substantially equivalent to parental and transgenic controls

Recombinant protease inhibitors represent useful tools for the development of insect-resistant transgenic crops, but questions have been raised in recent years about the impact of these proteins on endogenous proteases and chemical composition of derived food products. In this study, we performed a detailed compositional analysis of tubers from potato lines expressing the broad-spectrum inhibitor of Ser and Asp proteases, tomato cathepsin D inhibitor (SlCDI), to detect possible unintended effects on tuber composition. A compositional analysis of key nutrients and toxic chemicals was carried out with tubers of SlCDI-expressing and control (comparator) lines, followed by a two-dimensional gel electrophoresis (2-DE) proteomic profiling of total and allergenic proteins to detect eventual effects at the proteome level. No significant differences were observed among control and SlCDI-expressing lines for most chemicals assayed, in line with the very low abundance of SlCDI in tubers. Likewise, proteins detected after 2-DE showed no quantitative variation among the lines, except for a few proteins in some control and test lines, independent of slcdi transgene expression. Components of the patatin storage protein complex and Kunitz protease inhibitors immunodetected after 2-DE showed unaltered deposition patterns in SlCDI-expressing lines, clearly suggesting a null impact of slcdi on the intrinsic allergenic potential of potato tubers. These data suggest, overall, a null impact of slcdi expression on tuber composition and substantial equivalence between comparator and SlCDI-expressing tubers despite reported effects on leaf protein catabolism. They also illustrate the usefulness of proteomics as a tool to assess the authenticity of foods derived from novel-generation transgenic plants.

On-chip detection of low-molecular-weight recombinant proteins in plant crude extracts by SELDI-TOF MS

This chapter presents a general procedure for the on-chip detection and quantitation of low-molecular-weight recombinant proteins in transgenic plant crude extracts by surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF MS). A protocol is first described to detect the protein of interest in crude protein extracts of transgenic plant lines, by differential protein mapping against similar extracts from a control, nontransgenic line. A complementary protocol is then presented to generate a standard curve with the SELDI system, allowing the protein to be quantified in different transgenic lines. Overall, this procedure may be carried out within a few hours, without the need for prior purification or enrichment of the recombinant protein.

Targeting and post-translational processing of human alpha1-antichymotrypsin in BY-2 tobacco cultured cells

The post-translational processing of human alpha(1)-antichymotrypsin (AACT) in Bright Yellow-2 (BY-2) tobacco cells was assessed in relation to the cellular compartment targeted for accumulation. As determined by pulse-chase labelling experiments and immunofluorescence microscopy, AACT sent to the vacuole or the endoplasmic reticulum (ER) was found mainly in the culture medium, similar to a secreted form targeted to the apoplast. Unexpectedly, AACT expressed in the cytosol was found in the nucleus under a stable, non-glycosylated form, in contrast with secreted variants undergoing multiple post-translational modifications during their transit through the secretory pathway. All secreted forms of AACT were N-glycosylated, with the presence of complex glycans as observed naturally on human AACT. Proteolytic trimming was also observed for all secreted variants, both during their intracellular transit and after their secretion in the culture medium. Overall, the targeting of human AACT to different compartments of BY-2 tobacco cells led to the production of two protein products: (i) a stable, non-glycosylated protein accumulated in the nucleus; and (ii) a heterogeneous mixture of secreted variants resulting from post-translational N-glycosylation and proteolytic processing. Overall, these data suggest that AACT is sensitive to resident proteases in the ER, the Golgi and/or the apoplast, and that the production of intact AACT in the plant secretory pathway will require innovative approaches to protect its structural integrity in vivo. Studies are now needed to assess the activity of the different AACT variants, and to identify the molecular determinants for the nuclear localization of AACT expressed in the cytosol.

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.

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.

A case study for plant-made pharmaceuticals comparing different plant expression and production systems

Over the last decade, plant-based production of pharmaceuticals has made remarkable progress as the expression of a diverse set of proteins has been demonstrated in a range of plant crops. Although the commercial exploitation is still pending, today various plant-based expression technologies have reached significant milestones through clinical testing in humans. Each of the protein manufacturing platforms in plants has specific benefits and drawbacks. We have engaged in comparing some of these production systems with respect to their performance: protein yield and quality. Using a specific tester protein (aprotinin), it was shown that functional aprotinin can be manufactured in plants in substantial amounts, as illustrated in this chapter.

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.